Adding yaml as option for generator

New installations will get :yaml as it's default with openapi 3 as the
version. Old installations will have the key missing and will default
to :json with an easy upgrade path.

rswag-specs/lib/generators/rswag/specs/install/templates/swagger_helper.rb

@@ -13,8 +13,8 @@ RSpec.configure do |config|
   # document below. You can override this behavior by adding a swagger_doc tag to the
   # the root example_group in your specs, e.g. describe '...', swagger_doc: 'v2/swagger.json'
   config.swagger_docs = {
-    'v1/swagger.json' => {
-      swagger: '2.0',
+    'v1/swagger.yaml' => {
+      openapi: '3.0.1',
       info: {
         title: 'API V1',
         version: 'v1'
@@ -22,4 +22,10 @@ RSpec.configure do |config|
       paths: {}
     }
   }
+
+  # Specify the format of the output Swagger file when running 'rswag:specs:swaggerize'.
+  # The swagger_docs configuration option has the filename including format in
+  # the key, this may want to be changed to avoid putting yaml in json files.
+  # Defaults to json. Accepts ':json' and ':yaml'.
+  config.swagger_format = :yaml
 end