remove mathmlore class from mixing example (upsets w3c validator)

Author: davidcarlisle

src/mixing.html

@@ -300,7 +300,7 @@ <h4 id="mixing_semantics_description">Description</h4>
     rendering of its first child.  A renderer may use the information contained
     in the annotations to customize its rendering of the annotated element.</p>
 
-    <div class="example mathml mmlcore">
+    <div class="example mml4c">
       <pre>
         &lt;semantics&gt;
           &lt;mrow&gt;
@@ -859,7 +859,7 @@ <h4 id="mixing_top_level">Top-level Parallel Markup</h4>
      in this way.
     </p>
 
-    <div class="example mathml mmlcore">
+    <div class="example mml4c">
      <pre>
        &lt;semantics&gt;
          &lt;mrow&gt;
@@ -913,7 +913,7 @@ <h4 id="mixing_cr">Parallel Markup via Cross-References</h4>
       </math>.
     </p>
 
-    <div class="example mathml mmlcore">
+    <div class="example mml4c">
      <pre>
        &lt;semantics&gt;
          &lt;mrow id="E"&gt;