throttle: Update throttle-groups.c documentation

There used to be throttle_timers_{detach,attach}_aio_context() calls
in bdrv_set_aio_context(), but since 7ca7f0f6
they are now in blk_set_aio_context().
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/throttle-groups.c

@@ -49,7 +49,7 @@
  * Again, all this is handled internally and is mostly transparent to
  * the outside. The 'throttle_timers' field however has an additional
  * constraint because it may be temporarily invalid (see for example
- * bdrv_set_aio_context()). Therefore in this file a thread will
+ * blk_set_aio_context()). Therefore in this file a thread will
  * access some other BlockBackend's timers only after verifying that
  * that BlockBackend has throttled requests in the queue.
  */