Add double negation example to "Boolean Default"

Closes jquery#170

Author: kswedberg

pages/Types.html

@@ -144,6 +144,7 @@ <h3 id="Boolean_Default_2"> Boolean Default </h3>
 <p>If a number is zero, it defaults to false:
 </p>
 <pre><code data-lang="javascript">!0 // true
+!!0 // false (Double negation: Since "not 0" is true, negating that makes it false)
 !1 // false
 !-1 // false
 </code></pre>