Change platform mswin to windows

Author: ima1zumi

Gemfile

@@ -20,7 +20,7 @@ gem "test-unit-ruby-core"
 gem "rubocop"
 
 gem "tracer" if !is_truffleruby
-gem "debug", github: "ruby/debug", platforms: [:mri, :mswin]
+gem "debug", github: "ruby/debug", platforms: [:mri, :windows]
 
 gem "rdoc", ">= 6.11.0"