docs: Typo fix in live disk backup

Add in the missing space in the section header.

Fixes: 1084159b31 ("qapi: deprecate drive-backup", v6.2.0)
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/interop/live-block-operations.rst

@@ -931,8 +931,8 @@ Shutdown the guest, by issuing the ``quit`` QMP command::
     }
 
 
-Live disk backup --- ``blockdev-backup`` and the deprecated``drive-backup``
----------------------------------------------------------------------------
+Live disk backup --- ``blockdev-backup`` and the deprecated ``drive-backup``
+----------------------------------------------------------------------------
 
 The ``blockdev-backup`` (and the deprecated ``drive-backup``) allows
 you to create a point-in-time snapshot.