update invalid link. repo moved. (#3784)

Author: erichiller

xml/System.Diagnostics/DiagnosticSource.xml

@@ -21,7 +21,7 @@
   <Interfaces />
   <Docs>
     <summary>An abstract class that allows code to be instrumented for production-time logging of rich data payloads for consumption within the process that was instrumented.</summary>
-    <remarks>For more information, see [DiagnosticSource User's Guide](https://github.com/dotnet/corefx/blob/master/src/System.Diagnostics.DiagnosticSource/src/DiagnosticSourceUsersGuide.md).</remarks>
+    <remarks>For more information, see [DiagnosticSource User's Guide](https://github.com/dotnet/runtime/blob/master/src/libraries/System.Diagnostics.DiagnosticSource/src/DiagnosticSourceUsersGuide.md).</remarks>
   </Docs>
   <Members>
     <Member MemberName=".ctor">