Fixed a typo in the setSubPixelShift in the FXAA filter that was causing a crash

jme3-effects/src/main/java/com/jme3/post/filters/FXAAFilter.java

@@ -85,7 +85,7 @@ public class FXAAFilter extends Filter {
     public void setSubPixelShift(float subPixelShift) {
         this.subPixelShift = subPixelShift;
         if (material != null) {
-            material.setFloat("SubPixelShif", this.subPixelShift);
+            material.setFloat("SubPixelShift", this.subPixelShift);
         }
     }