media: doc: dev-encoder: Fixup type for ENUM_FRAMEINTERVALS

The doc mistakently mentions v4l2_frmsizeenum as the struct type passed
to VIDIOC_ENUM_FRAMEINTERVALS, while it's actually v4l2_frmivalenum that
should be used.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>