Added assets:precompile in upgrade guide 5.4 -> 6.0

doc/update/5.4-to-6.0.md

@@ -71,6 +71,8 @@ sudo -u git -H bundle exec rake migrate_global_projects RAILS_ENV=production
 sudo -u git -H bundle exec rake migrate_keys RAILS_ENV=production
 sudo -u git -H bundle exec rake migrate_inline_notes RAILS_ENV=production
+sudo -u git -H bundle exec rake assets:precompile RAILS_ENV=production
+
 ```
 ### 6. Update config files