sdp: fix wrong error message for missing clock-rate in caps

sdp: fix wrong error message for missing clock-rate in caps

When using `gst_sdp_media_set_media_from_caps` on `application/x-rtp` caps
without `clock-rate` it wrongly reports missing payload type even if `payload`
is present in the caps.

This seems to be a copy&paste error from the error message for missing payload
type.

When using payload=10, both `clock-rate` and some other media properties are
defined by the RTP standard so I was wondering whether I could omit `clock-rate`
and was confused about the error message.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5252>