doc improvement about alternative sollution when path '-' is deprecated

[xinminglai]

Problem or new feature

accorrding to https://docs.raku.org/type/IO/Handle "As of 6.d language version, the use of path '-' is deprecated and it will be removed in future language versions entirely."
The doc lacks info about the alternative way to do the same with '-'

Suggestions

lizmat suggests IO::Handle.new(path => IO::Special.new("STDIN"))
So I think it's good to have explanation for this.