fix typo

http://visionmedia.github.io/page.js/#contextquerystring

page/page.d.ts

@@ -188,7 +188,7 @@ declare module PageJS {
         /**
          *  Query string void of leading ? such as "foo=bar", defaults to "".
          */
-        queryString: string;
+        querystring: string;
         /**
          *  The pathname void of query string "/login".
          */