documentation: Removed a line about python version requirements of training script which can misguide users.

[rsareddy0329]

Issue #, if available:
#2826

Description of changes:
Removed a line about python version requirements of training script which can misguide users. It is implicit that the Training script can be of latest version based on the framework_version and supported py_version of the container that will be provided here: https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/pytorch/estimator.py#L144 while declaring the estimator.

Testing done:
yes, unit testing is done.

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