Fix error "Address already in use".

Created by: hiroponz

Some users have faced the same trouble. See #3663 (closed).

doc/update/5.0-to-5.1.md

@@ -19,6 +19,7 @@ sudo -u git -H git checkout 5-1-stable
 ### 3. Install libs, migrations etc
 ```
+sudo rm tmp/sockets/gitlab.socket
 sudo -u git -H cp config/puma.rb.example config/puma.rb