docs(test): drop https:// from regression-test comment

Addresses Qodo: PR Compliance ID 10 wants protocol-independent URLs
in new comments. Use the GitHub shorthand `#7138`
instead of a full https:// link.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

src/tests/backend/specs/settings.ts

@@ -148,7 +148,7 @@ describe(__filename, function () {
     });
   });
 
-  // Regression test for https://github.com/ether/etherpad/issues/7138.
+  // Regression test for ether/etherpad#7138.
   // padOptions.fadeInactiveAuthorColors must default to true so existing
   // installations keep the legacy fade-on-inactive behavior, and must be
   // overridable via PAD_OPTIONS_FADE_INACTIVE_AUTHOR_COLORS in docker.