Use the --purge flag in doc/install/kubernetes/gitlab_omnibus.md

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6101.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/install/kubernetes/gitlab_omnibus.md

@@ -228,7 +228,7 @@ helm upgrade gitlab --set gitlab=ee,gitlabEEImage=gitlab/gitlab-ee:9.5.5-ee.0 gi
 To uninstall the GitLab Chart, run the following:
 
 ```bash
-helm delete gitlab
+helm delete --purge gitlab
 ```
 
 ## Troubleshooting