Add command for updating the init.d script

doc/update/6.3-to-6.4.md

@@ -44,6 +44,9 @@ sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
 # Clean up assets and cache
 sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
+
+# Update init.d script
+sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
 ```
 ### 5. Start application