media: pixfmt-yuv-planar.rst: fix PIX_FMT labels

Two labels used _ instead of - so were never found and one new PIX_FMT was
missing the label altogether. This led to these warnings:

Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-pix-fmt-nv12m-8l128
Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-pix-fmt-nv12m-10be-8l128
Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-pix-fmt-mm21

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 72a74c8f ("media: add nv12m_8l128 and nv12m_10be_8l128 video format.")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>