qemu-img: Fix doc typo for 'bitmap' subcommand

Prefer a consistent naming for the --merge argument.

Fixes: 3b51ab4bf
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200529144527.1943527-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

docs/tools/qemu-img.rst

@@ -300,7 +300,7 @@ Command description:
 
   ``--disable`` to change *BITMAP* to stop recording future edits.
 
-  ``--merge`` to merge the contents of *SOURCE_BITMAP* into *BITMAP*.
+  ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
 
   Additional options include ``-g`` which sets a non-default
   *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an