Update casing of 'OmniAuth' to match that of official OmniAuth documentation

lib/generators/templates/devise.rb

@@ -257,7 +257,7 @@ Devise.setup do |config|
   # The router that invoked `devise_for`, in the example above, would be:
   # config.router_name = :my_engine
   #
-  # When using omniauth, Devise cannot automatically set Omniauth path,
+  # When using OmniAuth, Devise cannot automatically set OmniAuth path,
   # so you need to do it manually. For the users scope, it would be:
   # config.omniauth_path_prefix = '/my_engine/users/auth'
 end