doc: fix "hashOwnProperty" typo in querystring

querystring subsystem docs referred to `obj.hashOwnProperty()`
which is non-existent. Corrected to `obj.hasOwnProperty()`.

doc/api/querystring.md

@@ -58,7 +58,7 @@ For example, the query string `'foo=bar&abc=xyz&abc=123'` is parsed into:
 
 *Note*: The object returned by the `querystring.parse()` method _does not_
 prototypically extend from the JavaScript `Object`. This means that the
-typical `Object` methods such as `obj.toString()`, `obj.hashOwnProperty()`,
+typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`,
 and others are not defined and *will not work*.
 
 By default, percent-encoded characters within the query string will be assumed