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

Documentation/sound/alsa-configuration.rst

@@ -2261,6 +2261,7 @@ delayed_register
 quirk_flags
     Contains the bit flags for various device specific workarounds.
     Applied to the corresponding card index.
+
         * bit 0: Skip reading sample rate for devices
         * bit 1: Create Media Controller API entries
         * bit 2: Allow alignment on audio sub-slot at transfer