docs: fix incorrect sample code in ajax.rst

[kenjis]

Description
It is not valid that "application/json" and "GET".
The HTTP method names are case-sensitive, and it is not "get" but "GET".

The method token is case-sensitive because it might be used as a gateway to object-based systems with case-sensitive method names. By convention, standardized methods are defined in all-uppercase US-ASCII letters.
https://www.rfc-editor.org/rfc/rfc9110#name-overview

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