Merge branch 'dmishunov-master-patch-20941' into 'master'

doc: Updated the structure of User Timing section

See merge request gitlab-org/gitlab!49809

doc/development/fe_guide/performance.md

@@ -182,7 +182,7 @@ To access stored measurements, you can use either:
   performance.getEntriesByType('measure');
   ```
 
-## Naming convention
+### Naming convention
 
 All the marks and measures should be instantiated with the constants from
 `app/assets/javascripts/performance/constants.js`. When you’re ready to add a new mark’s or