ALSA: Fix typos in alsa-driver-api.xml

This patch fixed 2 typos in DocBook/alsa-driver-api.xml.
It is because this file is generated by make xmldocs,
I have to fix typos within source files.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>