Reword sentence about the mandatory type field

Author: duncan255

index.html

@@ -806,7 +806,7 @@ <h3 id="message-format">Message format</h3>
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                 <td>content</td>
-                <td>Free-form JSON. It is up to applications to interpret what it means. A mandatory <em>type</em> field, a string, between 3 and 52 utf8 chars long (inclusive). This is used so that applications can easily filter out message types they don’t understand.</td>
+                <td>Dictionary containing free-form data for applications to interpret, plus a mandatory <em>type</em> field. The <em>type</em> field allows applications to filter out message types they don’t understand and must be a string between 3 and 52 characters long (inclusive).</td>
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         </table>