git-maintenance.txt: add missing word

Add a missing "a" before "bunch".

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-maintenance.txt

@@ -101,7 +101,7 @@ This is done to avoid disrupting the remote-tracking branches. The end users
 expect these refs to stay unmoved unless they initiate a fetch.  With prefetch
 task, however, the objects necessary to complete a later real fetch would
 already be obtained, so the real fetch would go faster.  In the ideal case,
-it will just become an update to bunch of remote-tracking branches without
+it will just become an update to a bunch of remote-tracking branches without
 any object transfer.
 
 gc::