`GrapeSwagger::Errors::UnregisteredParser` in example

[Masayoshi Takahashi]

When I run the code in example and try to get swagger_doc, I get an error.

Steps to reproduce

$ ruby -v
ruby 2.7.5p203 (2021-11-24 revision f69aeb8314) [arm64-darwin21]
$ cd example
$ bundle install
$ bundle exec rackup

And when I accessed http://localhost:9292/, I got the following screen:

It found similar issues #424, #548 and #693 . I understand you do not want to add require "grape-swagger-entity" explicitly.
If so, I think we can add Bundler.require. I am going to write a pull request later and would appreciate your review.