Update jsguide.html

Issue google#656 -In  javascript guide , 3.3.3 is showing up in the table of contents as it is inside the <h3>.So updated it with h4 tag

jsguide.html

@@ -186,7 +186,7 @@ <h4 id="file-declare-legacy-namespace">3.3.2 <code>goog.module.declareLegacyName
 <code>goog.module('parent.child');</code> cannot both exist safely, nor can
 <code>goog.module('parent');</code> and <code>goog.module('parent.child.grandchild');</code>).</p>
 
-<h3 id="file-goog-module-exports">3.3.3 <code>goog.module</code> Exports</h3>
+<h4 id="file-goog-module-exports">3.3.3 <code>goog.module</code> Exports</h4>
 
 <p>Classes, enums, functions, constants, and other symbols are exported using the
 <code>exports</code> object. Exported symbols may be defined directly on the <code>exports</code>