Specify flavor to use for RE2 syntax testing

doc/user/packages/container_registry/reduce_container_registry_storage.md

@@ -281,7 +281,7 @@ Here are some other options you can use to reduce the Container Registry storage
 
 If you see this error message, check the regex patterns to ensure they are valid.
 
-GitLab uses [RE2 syntax](https://github.com/google/re2/wiki/Syntax) for regular expressions in the cleanup policy. You can test them with the [regex101 regex tester](https://regex101.com/).
+GitLab uses [RE2 syntax](https://github.com/google/re2/wiki/Syntax) for regular expressions in the cleanup policy. You can test them with the [regex101 regex tester](https://regex101.com/) using the Golang flavor.
 View some common [regex pattern examples](#regex-pattern-examples).
 
 ### The cleanup policy doesn't delete any tags