csharp: update GH link after branch rename

The dotnet project switched from master to main.

csharp-style.md

@@ -12,7 +12,7 @@ Naming rules follow
 [Microsoft's C# naming guidelines](https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/naming-guidelines).
 Where Microsoft's naming guidelines are unspecified (e.g. private and local
 variables), rules are taken from the
-[CoreFX C# coding guidelines](https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/coding-style.md)
+[CoreFX C# coding guidelines](https://github.com/dotnet/runtime/blob/HEAD/docs/coding-guidelines/coding-style.md)
 
 Rule summary: