start gitlab after mysql tweaks

doc/update/7.3-to-7.4.md

@@ -167,6 +167,10 @@ mysql> \q
 #   Set production -> password: the password your replaced $password with earlier
 sudo -u git -H editor /home/git/gitlab/config/database.yml
 
+# Start GitLab
+sudo service gitlab start
+sudo service nginx restart
+
 # Run thorough check
 sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
 ```