[ci skip] Add bundle exec to test run command

Author: yui-knk

guides/source/contributing_to_ruby_on_rails.md

@@ -334,7 +334,7 @@ will now run the four of them in turn.
 You can also run any single test separately:
 
 ```bash
-$ ARCONN=sqlite3 ruby -Itest test/cases/associations/has_many_associations_test.rb
+$ ARCONN=sqlite3 bundle exec ruby -Itest test/cases/associations/has_many_associations_test.rb
 ```
 
 To run a single test against all adapters, use: