Prefix the command to execute a single test in CONTRIBUTING.md with bundle exec (#3187)

Author: marcrohloff

CONTRIBUTING.md

@@ -48,5 +48,5 @@ bundle exec rake test TEST=test/rake_tasks_test.rb
 #### 4.2 Run a single ruby test
 
 ```
-ruby -I test test/rake_tasks_test.rb -n test_rake_webpacker_install
+bundle exec ruby -I test test/rake_tasks_test.rb -n test_rake_webpacker_install
 ```