Update PR template section about API breaking changes (#6723)

The `breaking change` label doesn't seem to exist. Also, a contributor
without triage GitHub permission level cannot set labels. Update the
instructions what to do on breaking API changes.

.github/pull_request_template.md

@@ -27,5 +27,5 @@ If there are user-facing changes then we may require documentation to be updated
 -->
 
 <!---
-If there are any breaking changes to public APIs, please add the `breaking change` label.
+If there are any breaking changes to public APIs, please call them out.
 -->