refactor(onboarding): consistent header depth

lib/workers/repository/onboarding/pr/index.js

@@ -41,7 +41,7 @@ You can post questions in [our Config Help repository](https://github.com/renova
     prBody =
       prBody.replace(
         '{{PACKAGE FILES}}',
-        '## Detected Package Files\n\n' + files.join('\n')
+        '### Detected Package Files\n\n' + files.join('\n')
       ) + '\n';
   } else {
     prBody = prBody.replace('{{PACKAGE FILES}}\n', '');