rtmpproto: Fix a typo in a comment

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

libavformat/rtmpproto.c

@@ -2974,7 +2974,7 @@ static int rtmp_write(URLContext *s, const uint8_t *buf, int size)
                                                      &rt->nb_prev_pkt[1],
                                                      channel)) < 0)
                     return ret;
-                // Force sending a full 12 bytes header by cleaing the
+                // Force sending a full 12 bytes header by clearing the
                 // channel id, to make it not match a potential earlier
                 // packet in the same channel.
                 rt->prev_pkt[1][channel].channel_id = 0;