Update NEW_TENANT.md
Update NEW_TENANT.md

instead of using an *absolute* path, use `run-as-user.sh`
instead of `/app/run-as-user.sh`


you need to understand what's going on there ...

if you execute `docker ps` you'll see that you're running `run-as-user.sh`
`run-as-user.sh` is a _shared_ script across different containers
that's why was removed from the `akvo-lumen` repo, and placed in a _proper_ place
that proper place is `/usr/local/bin`
https://github.com/akvo/akvo-dockerfiles/blob/master/lumen-backend-dev/Dockerfile#L5

you can always check the base images in the `akvo-dockerfiles` repo
akvo/akvo-lumen,akvo/akvo-lumen,akvo/akvo-dash,akvo/akvo-dash,akvo/akvo-dash