FIELD OF THE INVENTION The present invention, in general, relates to the shape of the tooth-profile of a shaft used for torque transmission. More specifically, it relates to the geometrical design of the tooth of a shaft in a twin-screw extruder. The design ensures almost nil stress concentration on the tooth while transferring the torque from one rotating member to another in the twin-screw extruder. BACKGROUND Torque transmitting systems are ubiquitous in all forms of mechanical devices. All torque transmitting systems must have a driving member and a driven member. Conventionally, torque is transmitted from the driving member to the driven member through means of key-ways, serrations, spline-profile teeth and the like. In case of spline-profile teeth the stress developed as a result of torque transmission, is not well distributed in the driving member and the driven member due to discontinuities in the spline-profile teeth. As a result, stress levels are high in certain regions of spline-profile teeth. This phenomenon is known as stress concentration. Stress concentration occurs at those regions of the tooth spline-profile teeth where there are discontinuities, making such regions susceptible to failure. This stress concentration limits the torque transmitting capacity of the torque transmitting system. In case of twin-screw extruders, driven members associated with twin-screw extruders will be hereinafter referred to as extruder elements and driving members associated with twin-screw extruders will be hereinafter referred to as extruder shafts. For better and efficient compounding of polymers in twin-screw extruders with a given extruder geometry, the extruder shafts need to be bigger, which results in thinner cross-section for the extruder elements. The problem of stress-concentration multiplies in case of thin walled driven members, i.e., extruder element having thin wall thickness. For extruder elements having wall thickness less than 2-3 mm, stress concentration dramatically reduces stress bearing capacity. This limits the torque transmitting capacity of the extruder elements, thereby reducing the efficiency of the twin-screw extruder. Efforts have been made to increase the torque transmitting capacity of the torque transmitting systems by designing various spline-profile teeth. Spline-profiles employed in the prior art include the straight-sided spline with polygonal tooth profile, the involute tooth profiled spline, and the cycloidal tooth profiled spline. All the profiles employed earlier have failed to effectively remove the stress concentration. Many of the spline-profiles disclosed in the prior art have discontinuities in the profile, which is the prime reason for stress concentration. Even the spline-profiles which have no discontinuities do not completely eliminate stress concentration. For example, US patent no. US5716159 titled "Spline and spline for rotary elements of multi spindle extruder" discloses a system which uses an elliptical spline-profile for tooth of extruder elements of a multi spindle extruder. The spline-profile has a semi-elliptical profile for its crest and a similar semi-elliptical profile for the trough. However, in the multi spindle extruder disclosed in the patent, the point where two semi-elliptical parts meet, forms a discontinuity and may lead to stress concentration. This again limits the torque transmitting capacity of the shaft. It is therefore necessary to have a torque transmitting system which has stress-concentration free profile tooth so that there is improved torque transmitting capacity of the shaft for a given diameter. SUMMARY The present invention discloses a tooth profile for improved torque transmitting capacity and for smooth transmission of torque from the driving member to the driven member in a torque transmitting system. The present invention is accomplished by a spline-profile tooth having a :orque transmitting system comprising a driving member having stress-:oncentration-free-profile teeth on its outer surface. The system further has a driven nember having stress-concentration-free-profile teeth conjugate to the stress-:oncentration-free-profile teeth of the driving member for transmitting torque :herebetween. The stress-concentration-free-profile teeth of the driving member and the driven member have a tooth profile defined in a plane of r-0 polar coordinati system by the mathematical expression for a half curve being defined by r=h[G/p - l/2TT(sin (2TT9/P))L in a defined area of 0