ALSA: doc: Fix indentation warning

Fix a trivial warning for the indentation by putting an empty line:
  Documentation/sound/alsa-configuration.rst:2258: WARNING: Unexpected indentation.

Fixes: a39978ed ("ALSA: doc: Add the description of quirk_flags option for snd-usb-audio")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20210823113518.30134-1-tiwai@suse.de


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