Fix typo in smtp_settings.rb.sample

Created by: damselem

config/initializers/smtp_settings.rb.sample

 # To enable smtp email delivery for your GitLab instance do next: 
 # 1. Change config/environments/production.rb to use smtp
 #    config.action_mailer.delivery_method = :smtp
-# 2. Rename this file to smpt_settings.rb
+# 2. Rename this file to smtp_settings.rb
 # 3. Edit settings inside this file
 # 4. Restart GitLab instance
 #