improve help info

@@ -221,7 +221,9 @@ void load_codec_param()
 int main(int argc, char * argv[])
 {
 	if (argc < 2) {
-		printf("%s record_info_filename [-p]");
+		printf("   run ffmpeg to merge audio and video files according to the record info file,\nusage:");
+		printf("\n   %s record_info_filename [-p]", argv[0]);
+		printf("\n   -p :only print the command,don't run ffmpeg");
 		return -1;
 	}