doc/filters: fix grammar mistake in setfield documentation

Spotted-by: Tim Nicholson <nichot20@yahoo.com>

doc/filters.texi

@@ -3712,8 +3712,8 @@ corresponding property, which affects how the frame is treated by
 following filters (e.g. @code{fieldorder} or @code{yadif}).
 
 This filter accepts a single option @option{mode}, which can be
-specified either by setting @code{mode=VALUE} either setting the
-value alone. Available values are:
+specified either by setting @code{mode=VALUE} or setting the value
+alone. Available values are:
 
 @table @samp
 @item auto