Fixed markdown error in 7.3-to-7.4 update guide

Just fixed a minor bug in the markdown notation

doc/update/7.3-to-7.4.md

@@ -9,6 +9,7 @@
 ```bash
 cd /home/git/gitlab
 sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
 
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