Fix return type of Process.detach to return Process::Waiter

[pocke]

Process.detach returns a Process::Waiter, a subclass of Thread. But the RBS defines that it returns a Thread.

This PR fixes the return type to Process::Waiter.

Process.detach returns Process::Waiter since Ruby 2.2.

$ docker run -it --rm rubylang/all-ruby ./all-ruby -e 'p Process.detach(1).class'

(snip)

ruby-1.8.0            Thread
...
ruby-2.1.10           Thread
ruby-2.2.0-preview1   Process::Waiter
...
ruby-3.4.0-preview2   Process::Waiter