Documentation: replacing unhelpful link (#2309)

The link that was previously used here does not contain information about supported framework version of PyTorch. 
Adding a link to the DLC images, which contains all supported PT version. https://github.com/aws/deep-learning-containers/blob/master/available_images.md

Co-authored-by: Ahsan Khan <ahsan.al.zaki@gmail.com>

Author: TEChopra1000

src/sagemaker/pytorch/estimator.py

@@ -75,7 +75,7 @@ def __init__(
             framework_version (str): PyTorch version you want to use for
                 executing your model training code. Defaults to ``None``. Required unless
                 ``image_uri`` is provided. List of supported versions:
-                https://github.com/aws/sagemaker-python-sdk#pytorch-sagemaker-estimators.
+                https://github.com/aws/deep-learning-containers/blob/master/available_images.md.
             py_version (str): Python version you want to use for executing your
                 model training code. One of 'py2' or 'py3'. Defaults to ``None``. Required
                 unless ``image_uri`` is provided.