actually give command necessary to update unicorn

Give command to update unicorn.rb rather than just say you need to do it.

doc/update/7.3-to-7.4.md

@@ -93,8 +93,8 @@ git diff origin/7-3-stable:config/gitlab.yml.example origin/7-4-stable:config/gi
 #### Change timeout for unicorn
 
 ```
-# config/unicorn.rb
-timeout 60
+# set timeout to 60
+sudo -u git -H editor config/unicorn.rb
 ```
 
 #### Change nginx https settings