Update another RichTextFX-specific CSS property to use the -rtfx pref…

…ix; "-fill" reverted back to "-color"

Note: "-fill" was initially used  instead of "-color" because of JDK-8133685. Now that a different prefix is used, this is no longer necessary

richtextfx/src/main/java/org/fxmisc/richtext/TextExt.java

@@ -160,7 +160,7 @@ public ObjectProperty<Paint> backgroundFillProperty() {
     private static class StyleableProperties {
 
         private static final CssMetaData<TextExt, Paint> BACKGROUND_FILL = new CssMetaData<TextExt, Paint>(
-                "-fx-background-fill",
+                "-rtfx-background-color",
                 StyleConverter.getPaintConverter(),
                 Color.TRANSPARENT) {
             @Override