fix local markdown file links (#9384)

Fixes a few local links.

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Author: pq

CONTRIBUTING.md

@@ -152,11 +152,11 @@ not want to merge `upstream/master` or update your Flutter SDK version, instead
 
 ## Running and debugging DevTools
 
-Please see [DEBUGGING.md][] for guidance on running and debugging DevTools.
+Please see [DEBUGGING.md]() for guidance on running and debugging DevTools.
 
 ## Testing for DevTools
 
-Please see [TESTING.md][] for guidance on running and writing tests.
+Please see [TESTING.md]() for guidance on running and writing tests.
 
 ## Appendix