The link to the original ENTRYPOINT in pytorch_extending_our_containers.ipynb doesn't work

Hi, in [pytorch_extending_our_containers](https://github.com/awslabs/amazon-sagemaker-examples/blob/master/advanced_functionality/pytorch_extending_our_containers/pytorch_extending_our_containers.ipynb)

The link "here" doesn't work (404 page not found).

> The original `ENTRYPOINT` specified within the SageMaker PyTorch is [here](https://github.com/aws/sagemaker-pytorch-container/blob/master/docker/0.4.0/final/Dockerfile.cpu#L18)

Open advanced_functionality/pytorch_extending_our_containers/pytorch_extending_our_containers.ipynb and locate the link labeled "here" in the notebook text. Replace the broken reference to the original ENTRYPOINT with a working destination, then verify that the link no longer returns a 404 page.

Written by the indexing model from the issue text.