rtpenc: Fix some odd comments

Some weird comments stem from the fact that the rtpdec and rtpenc
code was shared earlier.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc.h

@@ -38,11 +38,9 @@ struct RTPMuxContext {
     int max_payload_size;
     int num_frames;
 
-    /* rtcp sender statistics receive */
+    /* rtcp sender statistics */
     int64_t last_rtcp_ntp_time;
     int64_t first_rtcp_ntp_time;
-
-    /* rtcp sender statistics */
     unsigned int packet_count;
     unsigned int octet_count;
     unsigned int last_octet_count;