[ExpressionLanguage] add missing argument

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | #4578
Hmm, I don't know why I created this code... It seems like completely wrong :)

Yes, let's change it to just `$expression = $language->parse('1 + 4', array());` and add a comment that this method returns a ``ParsedExpression``