docs: ABI: remove extension from sysfs-class-mic.txt

Files under ABI doesn't end with a .txt extension.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Link: https://lore.kernel.org/r/5113d1ae9d2dd3bc685bbb65655abd67e26f48c4.1562668471.git.mchehab+samsung@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>