[ci skip] Add `bundle exec` to Running a Single Test

guides/source/contributing_to_ruby_on_rails.md

@@ -295,7 +295,7 @@ You can run a single test through ruby. For instance:
 
 ```bash
 $ cd actionmailer
-$ ruby -w -Itest test/mail_layout_test.rb -n test_explicit_class_layout
+$ bundle exec ruby -w -Itest test/mail_layout_test.rb -n test_explicit_class_layout
 ```
 
 The `-n` option allows you to run a single method instead of the whole