Change header of section on replying to a non-discussion comment

doc/user/discussions/index.md

@@ -339,7 +339,7 @@ and push the suggested change directly into the codebase in the merge request's
 Custom commit messages will be introduced by
 [#54404](https://gitlab.com/gitlab-org/gitlab-ce/issues/54404).
 
-## Start a discussion from a non-discussion comment
+## Start a discussion by replying to a non-discussion comment
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/30299) in GitLab 11.9