Fix MathML rendering. Missed renaming an override method after my cleanup work in the FO tree.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@378471 13f79535-47bb-0310-9956-ffa450edef68

examples/mathml/src/org/apache/fop/mathml/MathMLElement.java

@@ -147,9 +147,9 @@ public class MathMLElement extends MathMLObj {
     }
 
     /**
-     * @see org.apache.fop.fo.XMLObj#getDocumentNamespace()
+     * @see org.apache.fop.fo.FONode#getNamespaceURI()
      */
-    public String getDocumentNamespace() {
+    public String getNamespaceURI() {
         if (svgDoc == null) {
             return MathMLElementMapping.NAMESPACE;
         }