audio: fix ALSA period-length typo in documentation

Fixes: f0b3d811 ("audio: -audiodev command line option: documentation")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190918095335.7646-4-stefanha@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>