Update Gemfile in buildpack author guide

- Ruby 3+ no longer includes webrick so it has to be
explicitly required in the Gemfile for the app to start
- See: https://bugs.ruby-lang.org/issues/17303

Signed-off-by: Tim Downey <[email protected]>

Author: tcdowney

content/docs/buildpack-author-guide/create-buildpack/setup-local-environment.md

@@ -46,11 +46,12 @@ Then, create a file called `ruby-sample-app/Gemfile`<!--+"{{open}}"+--> with the
 
 <!-- test:file=ruby-sample-app/Gemfile -->
 ```ruby
-source "https://rubygems.org"
+source 'https://rubygems.org'
 
 git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
 
-gem "sinatra"
+gem 'sinatra'
+gem 'webrick'
 ```
 
 Finally, make sure your local Docker daemon is running by executing: