media: media-types.rst: rename media-entity-type to media-entity-functions

The MEDIA_ENT_F_* defines refer to functions, not types. Update the
documentation accordingly.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>