media: media-types.rst: codec entities can have more than one source pad

Some decoders and encoders can potentially have more than one source pad,
so update the description to say 'at least one source pad'.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>