doc: fix default value of opts.decodeURIComponent

In the documentation for querystring.parse, the documentation mentions
that the default value for options.decodeURIComponent is the
decodeURIComponent function, but it's actually the querystring.unescape
function.

PR-URL: https://github.com/joyent/node/pull/9259
Reviewed-by: Julien Gilli <julien.gilli@joyent.com>

doc/api/querystring.markdown

@@ -42,7 +42,7 @@ characters.
 Options object may contain `maxKeys` property (equal to 1000 by default), it'll
 be used to limit processed keys. Set it to 0 to remove key count limitation.
 
-Options object may contain `decodeURIComponent` property (`decodeURIComponent` by default),
+Options object may contain `decodeURIComponent` property (`querystring.unescape` by default),
 it can be used to decode `non-utf8` encoding string if necessary.
 
 Example: