fftools/opt_common: add separator line to show_filters output

Add a separator line to show_filters() function output to maintain
consistent formatting with other show functions like show_codecs,
show_formats_devices, etc. This provides uniform formatting across
all command-line output functions for better readability and
parsing by external tools.

fftools/opt_common.c

@@ -809,7 +809,8 @@ int show_filters(void *optctx, const char *opt, const char *arg)
            "  A = Audio input/output\n"
            "  V = Video input/output\n"
            "  N = Dynamic number and/or type of input/output\n"
-           "  | = Source or sink filter\n");
+           "  | = Source or sink filter\n"
+           "  ------\n");
     while ((filter = av_filter_iterate(&opaque))) {
         descr_cur = descr;
         for (i = 0; i < 2; i++) {