Merge pull request rails#15667 from samdec/mailer_doc_bug_fix

fix bug in email with name example code

Author: sgrif

guides/source/action_mailer_basics.md

@@ -309,7 +309,7 @@ email address in the format `"Full Name <email>"`.
 ```ruby
 def welcome_email(user)
   @user = user
-  email_with_name = "#{@user.name} <#{@user.email}>"
+  email_with_name = %("#{@user.name}" <#{@user.email}>)
   mail(to: email_with_name, subject: 'Welcome to My Awesome Site')
 end
 ```