docs: fix doc comments for Router

[kenjis]

Description

• small fix

Checklist:

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

[kenjis]

It returns "Controller classname or Closure", but I don't know why this method needs to return. Because we have controllerName() for that purpose.