fix: CONTRIBUTING.md - broken link

Author: NikiforovAll

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 Contributing
 ============
 
-Please read [.NET Core Guidelines](https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/contributing.md) for more general information about coding styles, source structure, making pull requests, and more.
+Please read [.NET Core Guidelines](https://github.com/dotnet/runtime/blob/master/CONTRIBUTING.md) for more general information about coding styles, source structure, making pull requests, and more.
 While this project is in the early phases of development, some of the guidelines in this document -- such as API reviews -- do not yet apply as strongly.
 That said, please use open a GitHub to discuss first any API renames or changes before submitting PRs.