helptext: fix sentence in extensions documentation

Differential Revision: https://phab.mercurial-scm.org/D9366

mercurial/helptext/internals/extensions.txt

@@ -175,7 +175,7 b' repository configuration::'
 
 Be aware that ``uisetup`` in NOT the function to configure a ``ui`` instance.
 It's called only once per process, not per ``ui`` instance. Also, any changes
-to the ``ui`` may be discarded because the ``ui`` here temporarily loaded
+to the ``ui`` may be discarded because the ``ui`` here is a temporarily loaded
 local configuration. So, it's generally wrong to do `ui.setconfig()` in
 these callbacks. Notable exception is setting ``pre/post-<command>`` hooks
 and extending ``ui.__class__``.