Modifies generator and specs to look for openapi: 3.0.0 vs swagger 2.0

Renames rswag-api to rswag_api as that is preferred file naming convention in initializers per rubocop linting

rswag-api/spec/generators/rswag/api/install_generator_spec.rb

@@ -17,7 +17,7 @@ module Rswag
       end
 
       it 'installs the Rails initializer' do
-        assert_file('config/initializers/rswag-api.rb')
+        assert_file('config/initializers/rswag_api.rb')
       end
 
       # Don't know how to test this