Add a a reference to PrettyPrinter to code generation doc
Add a a reference to PrettyPrinter to code generation doc

The `$prettyPrinter` variable was undefined in the doc.

Also, the PrettyPrinter was mentioned as `default`, but it's `Standard`, actually.

etcinit/PHP-Parser,FireWalkerX/PHP-Parser,splp/PHP-Parser,schmittjoh/PHP-Parser,paulgav/PHP-Parser,djun-kim/PHP-Parser,kangkot/PHP-Parser,sdgdsffdsfff/PHP-Parser,cwi-swat/PHP-Parser,ovr/zephir-parser,paulgav/PHP-Parser,cwi-swat/PHP-Parser,mintbridge/PHP-Parser,Trismegiste/PHP-Parser,suralc/PHP-Parser,cookpan001/PHP-Parser,nikic/PHP-Parser,nikic/PHP-Parser