doc/ffprobe: clarify that the input file is not optional

Fix trac issue http://trac.ffmpeg.org/ticket/9562

doc/ffprobe.texi

@@ -12,7 +12,7 @@
 
 @chapter Synopsis
 
-ffprobe [@var{options}] [@file{input_url}]
+ffprobe [@var{options}] @file{input_url}
 
 @chapter Description
 @c man begin DESCRIPTION

fftools/ffprobe.c

@@ -3417,7 +3417,7 @@ end:
 static void show_usage(void)
 {
     av_log(NULL, AV_LOG_INFO, "Simple multimedia streams analyzer\n");
-    av_log(NULL, AV_LOG_INFO, "usage: %s [OPTIONS] [INPUT_FILE]\n", program_name);
+    av_log(NULL, AV_LOG_INFO, "usage: %s [OPTIONS] INPUT_FILE\n", program_name);
     av_log(NULL, AV_LOG_INFO, "\n");
 }