ALSA: doc: Fix PCM interface section typos

ALSA: doc: Fix PCM interface section typos

Fix two mistakes in the PCM interface section:
1/ Members of the snd_pcm_hardware structure are channels_{min,max}
   and not channel_{min,max} (mind the 's').
2/ Another sentence is incomplete as the reference to one structure
   member (period_bytes_max) is missing.

There is no relevant 'Fixes:' tag to apply as both typos predate the
Git era.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20230130162924.119389-1-miquel.raynal@bootlin.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>