Fix typography helpers "size" documentation

Incorrect classes on the example text had all the examples the same size. 
Fixed to apply the correct class on the examples.

docs/documentation/helpers/typography-helpers.html

@@ -82,7 +82,7 @@ breadcrumb:
         {% assign key = '$size-' | append: forloop.index %}
         <td><code>is-size-{{ forloop.index }}</code></td>
         <td><code>{{ initial_vars[key].value }}</code></td>
-        <td><span class="{{ initial_vars[key].value }}">Example</span></td>
+        <td><span class="is-size-{{ forloop.index }}">Example</span></td>
       </tr>
     {% endfor %}
   </tbody>