Fixing broken link to contribution guide.
Based on a similar issue in azure/azure-sdk-for-c
(https://github.com/Azure/azure-sdk-for-c/issues/1101), I assumed this
should point to
# Contributing to Azure Python SDK
If you would like to become an active contributor to this project please
-follow the instructions provided in [Microsoft Azure Projects Contribution Guidelines](https://azure.github.io/guidelines/).
+follow the instructions provided in [Microsoft Azure Projects Contribution Guidelines](https://opensource.microsoft.com/collaborate/).
## Building and Testing
