Fixed broken link in CONTRIBUTING.md (#9518)

Fixed link to PULL_REQUEST_TEMPLATE.md in CONTRIBUTING.md

.github/CONTRIBUTING.md

@@ -23,7 +23,7 @@ Follow the style used by the [.NET Foundation](https://github.com/dotnet/corefx/
 - We do not use the `private` keyword, as it is the default accessibility level in C#.
 - We use hard tabs over spaces.
 
-Read and follow our [Pull Request template](https://github.com/xamarin/Xamarin.Forms/blob/master/PULL_REQUEST_TEMPLATE.md).
+Read and follow our [Pull Request template](PULL_REQUEST_TEMPLATE.md).
 
 ### Pull Request Requirements