DAS plugins: fix broken link in writing_rules.md

Change-Id: Ic63fbc3b4c8ae9c1bff866ed32d0cd02eec55bfb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/456721
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analysis_server_plugin/doc/writing_rules.md

@@ -174,7 +174,7 @@ See [writing a plugin][] for information about the `Plugin` class.
 
 ## Testing an analysis rule
 
-Writing tests for an analysis rule is very easy, and is documented at [testing_rules][].
+Writing tests for an analysis rule is very easy, and is documented at [testing rules][].
 
 [writing a plugin]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server_plugin/doc/writing_a_plugin.md
-[testing rules]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server_plugin/doc/testing_rules.md
+[testing rules]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server_plugin/doc/testing_rules.md