doc/filters: Fix idet option name "rep_thres".

Reported by forum user vovcheck.

doc/filters.texi

@@ -8163,7 +8163,7 @@ The filter accepts the following options:
 Set interlacing threshold.
 @item prog_thres
 Set progressive threshold.
-@item repeat_thres
+@item rep_thres
 Threshold for repeated field detection.
 @item half_life
 Number of frames after which a given frame's contribution to the