media: pixfmt*.rst: replace a two dots by a comma

media: pixfmt*.rst: replace a two dots by a comma

On several tables, the color sample location table preamble is
written as:
	Color Sample Location..
Instead of:
	Color Sample Location:

I suspect that the repetition of such pattern was due to some
copy-and-paste (or perhaps some error during DocBook conversion).

Anyway, fix it.

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