ALSA: doc: Fix missing backquote in midi-2.0.rst

Fix the missing  missing backquote that caused a sphinx warning:
  Documentation/sound/designs/midi-2.0.rst:517: WARNING: Inline interpreted text or phrase reference start-string without end-string.

Fixes: e240cff9 ("ALSA: documentation: Add description for USB MIDI 2.0 gadget driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20230825152957.18c54ae2@canb.auug.org.au
Link: https://lore.kernel.org/r/20230825092351.11780-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/midi-2.0.rst

@@ -515,7 +515,7 @@ connection::
   % echo 2 > functions/midi2.usb0/ep.0/block.1/is_midi1
 
 For disabling the processing of UMP Stream messages in the gadget
-driver, pass `0` to `process_ump attribute in the top-level config::
+driver, pass `0` to `process_ump` attribute in the top-level config::
 
   % echo 0 > functions/midi2.usb0/process_ump