doc: fix header depth of util.isSymbol

PR-URL: https://github.com/nodejs/node/pull/7138
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/util.md

@@ -498,7 +498,7 @@ util.isString(5)
   // false
 ```
 
-## util.isSymbol(object)
+### util.isSymbol(object)
 
     Stability: 0 - Deprecated