fix javadoc

Author: rbri

src/main/java/com/gargoylesoftware/css/dom/RGBColorImpl.java

@@ -227,7 +227,7 @@ public CSSValueImpl getAlpha() {
 
     /**
      * Sets the alpha part to a new value.
-     * @param lightness the new CSSPrimitiveValue
+     * @param alpha the new CSSPrimitiveValue
      */
     public void setAlpha(final CSSValueImpl alpha) {
         alpha_ = alpha;