cosmetics: fix usefuLL --> usefuL typo

Originally committed as revision 8061 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtp.h

@@ -53,7 +53,7 @@ extern URLProtocol rtp_protocol;
 #define RTCP_TX_RATIO_NUM 5
 #define RTCP_TX_RATIO_DEN 1000
 
-/* Structure listing usefull vars to parse RTP packet payload*/
+/* Structure listing useful vars to parse RTP packet payload*/
 typedef struct rtp_payload_data_s
 {
     int sizelength;

libavutil/rational.h

@@ -56,7 +56,7 @@ static inline double av_q2d(AVRational a){
 
 /**
  * reduce a fraction.
- * this is usefull for framerate calculations
+ * This is useful for framerate calculations.
  * @param max the maximum allowed for dst_nom & dst_den
  * @return 1 if exact, 0 otherwise
  */