pixfmt-yuv-planar.rst: fix typo: 'Cr, Cr' -> 'Cr, Cb'

Several NV formats incorrectly described the Chroma order as
Cr, Cr when it actually is Cr, Cb.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Frank Barchard <fbarchard@google.com>