markdown_docs: Fix styling for in-app docs for code-blocks.

This wraps the `pre` with a `div.codehilite` so as to apply the
night mode styles.

templates/zerver/app/markdown_help.html

@@ -95,8 +95,10 @@
 def zulip():
     print "Zulip"
 ```</td>
-                    <td><pre>def zulip():
-    print "Zulip"</pre></td>
+                    <td>
+                        <div class="codehilite"><pre>def zulip():
+    print "Zulip"</pre></div>
+                    </td>
                     </tr>
                     <tr>
                     <td class="preserve_spaces">```python