[media] v4l: doc: Fix typo in vidioc-g-tuner.rst

This patch fix spelling typos found in vidioc-g-tuner.rst

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>