Fixing link to contribution guide (#466)

Fixes #465

CONTRIBUTING.md

@@ -4,7 +4,7 @@ Thank you for your interest in contributing to Azure SDK for iOS.
 
 - For reporting bugs, requesting features, or asking for support, please file an issue in the [issues](https://github.com/Azure/azure-sdk-for-ios/issues) section of the project.
 
-- If you would like to become an active contributor to this project please follow the instructions provided in [Microsoft Azure Projects Contribution Guidelines](http://azure.github.com/guidelines.html).
+- If you would like to become an active contributor to this project please follow the instructions provided in [Microsoft Azure Projects Contribution Guidelines](https://opensource.microsoft.com/collaborate).
 
 - To make code changes, or contribute something new, please follow the [GitHub Forks / Pull requests model](https://help.github.com/articles/fork-a-repo/): Fork the repo, make the change and propose it back by submitting a pull request.