Include the checkbox to select in docs

doc/user/project/merge_requests/allow_collaboration.md

@@ -28,11 +28,12 @@ source project and only lasts while the merge request is open. Once enabled,
 upstream members will also be able to retry the pipelines and jobs of the
 merge request:
 
-1. Enable the contribution while creating or editing a merge request.
+1. While creating or editing a merge request, select the checkbox **Allow
+   commits from members who can merge to the target branch**.
 
    ![Enable contribution](img/allow_collaboration.png)
 
-1. Once the merge request is created, you'll see that commits from members who
+1. Once the merge request is created, you can see that commits from members who
    can merge to the target branch are allowed.
 
    ![Check that contribution is enabled](img/allow_collaboration_after_save.png)