GitLab: take into account that one needs Developer access to assign a MR

wiki/src/contribute/merge_policy/submit.mdwn

@@ -49,8 +49,8 @@ To submit your branch for review:
       - Use the description to:
          - Summarize what problem this MR will fix, in terms of impact on users.
          - Reference the issues this MR will solve: `Closes #xxx, #yyyy`.
-      - If it's obvious to you who can, and should, review your branch: assign
-        the MR to that person.<br/>
+      - If it's obvious to you who can, and should, review your branch,
+        and you're allowed to assign MRs: assign the MR to that person.<br/>
         Else, leave the MR unassigned:
         the [[Foundations Team|working_together/roles/foundations_team]] will
         either handle the review themselves or help you find a suitable reviewer.