Rephrase line for clarity

This wording conveys the same meaning as the previous version, but
with simpler phrasing.

doc/user/project/repository/web_editor.md

@@ -108,7 +108,7 @@ You can see a **Create merge request** dropdown below the issue description.
 The **Create merge request** button doesn't display if:
 
 - A branch with the same name already exists.
-- The branch already has a referenced merge request.
+- A merge request already exists for this branch.
 - Your project has an active fork relationship.
 
 To make this button appear, one possible workaround is to