[doc] Outdated Entry in Hash

[Castone22]

https://github.com/ruby/rbs/blob/418f74d11ee0f02eea5efe3514057d1d629afc0c/stdlib/builtin/hash.rbs#L52-L61

Doc entry at the linked line isn't compliant with ruby 2.7 anymore... I'm not really sure what the best way to present the alternative would be.