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.