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/lib/generators/rswag/api/install/USAGE

@@ -5,4 +5,4 @@ Example:
     rails generate rswag:api:install
 
     This will create:
-        config/initializers/rswag-api.rb
+        config/initializers/rswag_api.rb

rswag-api/lib/generators/rswag/api/install/install_generator.rb

@@ -7,7 +7,7 @@ module Rswag
       source_root File.expand_path('../templates', __FILE__)
 
       def add_initializer
-        template('rswag-api.rb', 'config/initializers/rswag-api.rb')
+        template('rswag_api.rb', 'config/initializers/rswag_api.rb')
       end
 
       def add_routes

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

rswag-api/spec/rswag/api/fixtures/swagger/v1/swagger.json

@@ -1,5 +1,5 @@
 {
-  "swagger": "2.0",
+  "openapi": "3.0.0",
   "info": {
     "title": "API V1",
     "version": "v1"

rswag-api/spec/rswag/api/middleware_spec.rb

@@ -61,7 +61,7 @@ module Rswag
           it 'locates files at the provided swagger_root' do
             expect(response.length).to eql(3)
             expect(response[1]).to include( 'Content-Type' => 'application/json')
-            expect(response[2].join).to include('"swagger":"2.0"')
+            expect(response[2].join).to include('"openapi":"3.0.0"')
           end
         end