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>