Improve messages printed when there is no input/output file, making

them more grammatically correct and consistent with other equivalent
messages.

Originally committed as revision 15869 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3880,12 +3880,12 @@ int main(int argc, char **argv)
 
     /* file converter / grab */
     if (nb_output_files <= 0) {
-        fprintf(stderr, "Must supply at least one output file\n");
+        fprintf(stderr, "At least one output file must be specified\n");
         av_exit(1);
     }
 
     if (nb_input_files == 0) {
-        fprintf(stderr, "Must supply at least one input file\n");
+        fprintf(stderr, "At least one input file must be specified\n");
         av_exit(1);
     }