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

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: e240cff9e6e9 ("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.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>