avutil/opt: fix mis-alignment of option and constant values for filter help

Before:

overlay AVOptions:
  x                 <string>     ..FV....... set the x expression (default "0")
  y                 <string>     ..FV....... set the y expression (default "0")
  eof_action        <int>        ..FV....... Action to take when encountering EOF from secondary input  (from 0 to 2) (default repeat)
     repeat          0            ..FV....... Repeat the previous frame.
     endall          1            ..FV....... End both streams.
     pass            2            ..FV....... Pass through the main input.
  eval              <int>        ..FV....... specify when to evaluate expressions (from 0 to 1) (default frame)

After:
a
overlay AVOptions:
   x                 <string>     ..FV....... set the x expression (default "0")
   y                 <string>     ..FV....... set the y expression (default "0")
   eof_action        <int>        ..FV....... Action to take when encountering EOF from secondary input  (from 0 to 2) (default repeat)
     repeat          0            ..FV....... Repeat the previous frame.
     endall          1            ..FV....... End both streams.
     pass            2            ..FV....... Pass through the main input.
   eval              <int>        ..FV....... specify when to evaluate expressions (from 0 to 1) (default frame)

Signed-off-by: softworkz <softworkz@hotmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavutil/opt.c

@@ -1173,7 +1173,7 @@ static void opt_list(void *obj, void *av_log_obj, const char *unit,
             av_log(av_log_obj, AV_LOG_INFO, "     %-15s ", opt->name);
         else
             av_log(av_log_obj, AV_LOG_INFO, "  %s%-17s ",
-                   (opt->flags & AV_OPT_FLAG_FILTERING_PARAM) ? "" : "-",
+                   (opt->flags & AV_OPT_FLAG_FILTERING_PARAM) ? " " : "-",
                    opt->name);
 
         switch (opt->type) {