fftools/cmdutils: surround option arguments by <> in help output

Makes it more clear that they are placeholders for actual values.

fftools/cmdutils.c

@@ -132,10 +132,9 @@ void show_help_options(const OptionDef *options, const char *msg, int req_flags,
         else if (po->flags & OPT_FLAG_SPEC)
             av_strlcat(buf, "[:<spec>]", sizeof(buf));
 
-        if (po->argname) {
-            av_strlcat(buf, " ", sizeof(buf));
-            av_strlcat(buf, po->argname, sizeof(buf));
-        }
+        if (po->argname)
+            av_strlcatf(buf, sizeof(buf), " <%s>", po->argname);
+
         printf("-%-17s  %s\n", buf, po->help);
     }
     printf("\n");