CONTRIBUTING: Fix typos and spelling

CONTRIBUTING.rst

@@ -38,7 +38,7 @@ To contribute patches:
 #. Push the topic branch to a personal repository fork on GitLab.
 #. Create a GitLab Merge Request targeting the upstream ``master`` branch
    (even if the change is intended for merge to the ``release`` branch).
-   Check the box labelled "Allow commits from members who can merge to the
+   Check the box labeled "Allow commits from members who can merge to the
    target branch".  This will allow maintainers to make minor edits on your
    behalf.