Update README.md fixed Deadlink (#594)

fixed Contribution Guide link.
Same link as the next page.
https://github.com/Azure/azure-powershell/blob/main/README.md

README.md

@@ -17,5 +17,5 @@ We gladly accept community contributions.
 
 * Issues: Please report bugs using the Issues section of GitHub
 * Source Code Contributions:
- * Please follow the [Contribution Guidelines for Microsoft Azure](http://azure.github.io/guidelines.html) open source that details information on onboarding as a contributor
+ * Please follow the [Microsoft Azure Projects Contribution Guidelines](https://opensource.microsoft.com/collaborate) open source that details information on onboarding as a contributor
  * See [C# Coding Style](https://github.com/Azure/DotNetty/wiki/C%23-Coding-Style) for reference on coding style.