docs: fix incorrect sample code in restful.rst

[kenjis]

Description
Fixes #8248

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

[sammyskills]

I think it would be nice to also show the example using array callable syntax, where a namespace is specified or backward slash is appended, just like you have in the #8248 (comment)

[kenjis]

Added sample code:

[sammyskills]

Thank you @kenjis