reftests: I can't count in CSS

nth-child() is 1-indexed, not 0-indexed.

It doesn't matter for this test really, but better do it right to not confuse
poor developers who wonder why the first image is highlighted when nth-child(0)
clearly states "none".

testsuite/reftests/gtk-image-effect-inherit.css

@@ -1,11 +1,11 @@
-GtkBox > :nth-child(0) {
+GtkBox > :nth-child(1) {
   -gtk-image-effect: none;
 }
 
-GtkBox > :nth-child(1) {
+GtkBox > :nth-child(2) {
   -gtk-image-effect: highlight;
 }
 
-GtkBox > :nth-child(2) {
+GtkBox > :nth-child(3) {
   -gtk-image-effect: dim;
 }