remove mathml class from mixing example (upsets w3c validator)

Author: davidcarlisle

src/mixing.html

@@ -133,7 +133,7 @@ <h3 id="mixing_alternate_representations">Alternate representations</h3>
    </p>
 
    <!-- mml4c to prevent inlining -->
-   <div class="example mathml mml4c">
+   <div class="example mml4c">
      <pre>
        &lt;semantics&gt;
          &lt;apply&gt;
@@ -203,7 +203,7 @@ <h3 id="mixing_content_equiv">Content equivalents</h3>
 
 
    <!-- mml4c to prevent inlining -->
-   <div class="example mathml mml4c">
+   <div class="example mml4c">
      <pre>
        &lt;semantics&gt;
          &lt;mrow&gt;