Fix the sed command for increasing the backlog

doc/update/7.2-to-7.3.md

@@ -97,7 +97,7 @@ git diff origin/7-2-stable:config/gitlab.yml.example origin/7-3-stable:config/gi
 
 ```
 # Use the default Unicorn socket backlog value of 1024
-sudo -u git -H sed -i 's|^:backlog => 64|:backlog => 1024|' config/unicorn.rb
+sudo -u git -H sed -i 's/:backlog => 64/:backlog => 1024/' config/unicorn.rb
 ```
 
 * HTTP setups: Make `/etc/nginx/sites-available/nginx` the same as https://gitlab.com/gitlab-org/gitlab-ce/blob/7-3-stable/lib/support/nginx/gitlab but with your settings.