doc-rst: pixfmt-nv12m: fix conversion issues

The conversion added an empty column (probably, it was used on
DocBook just to increase spacing.

It also added an extra line on one of the texts, breaking
the original paragraph into two ones.

Remove them.

Finally, a space is required before :sub:, as otherwise it
won't display it right. Add it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>