ball: fix filter variable names

(cherry picked from commit 432a626a513e768023cfd5bf9fff735b146611fc)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/video_filter/ball.c

@@ -149,7 +149,7 @@ vlc_module_begin ()
 vlc_module_end ()
 
 static const char *const ppsz_filter_options[] = {
-    "ball-color", "ball-speed", "ball-size",
+    "color", "speed", "size",
     "gradient-threshold", "edge-visible", NULL
 };