docs: replace $response with $this->response in response.rst

[kenjis]

Description
The Response object is used mainly in controllers and the user guide explains that "It can be accessed through $this->response."

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide