3.0 - Fixed the docs for cakephp/cakephp#4816

See https://github.com/cakephp/cakephp/pull/4816#issuecomment-58018392
Signature of cache method is this `public function cache($key, $config = 'default')`. So we would have to call cache method from get() in two different ways (with and withoud the second parameter). Second option would be to hardcode the name of default cache (string `default`) into `Table` class, which is easy, but did not seem right.