doc/filters: add a curves example with psfile.

doc/filters.texi

@@ -2422,6 +2422,12 @@ Or simply:
 @example
 curves=vintage
 @end example
+
+@item
+Use a Photoshop preset and redefine the points of the green component:
+@example
+curves=psfile='MyCurvesPresets/purple.asv':green='0.45/0.53'
+@end example
 @end itemize
 
 @anchor{decimate}