media: libv4l-introduction.rst: fix undefined label

V4L2_PIX_FMT_HM12 was redefined to V4L2_PIX_FMT_NV12_16L16, so use the
new name in libv4l-introduction.rst.

This fixes this warning:

libv4l-introduction.rst:27: WARNING: undefined label: v4l2-pix-fmt-hm12

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>