Link to 'SageMaker Debugger's Hook' is broken in the readme

In the section [AWS Deep Learning Containers with Zero Code Change](https://github.com/awslabs/sagemaker-debugger#aws-deep-learning-containers-with-zero-code-change) there is a link to 'SageMaker Debugger's Hook' which currently points to the [glossary](https://github.com/awslabs/sagemaker-debugger/blob/master/api.md#glossary) which doesn't not exist anymore

Open the README section “AWS Deep Learning Containers with Zero Code Change” and inspect the “SageMaker Debugger's Hook” link. Replace the obsolete glossary destination with the correct existing documentation target, then verify that the link resolves.

Written by the indexing model from the issue text.