Fix documentation typo

## What does this MR do?
Fixes a typo in the documentation file `docs/configuration/advanced-configuration.md`

## Why was this MR needed?
The documentation file in question suggests running the command `sudo usermod -aG user docker`, which does not run when tested on Ubuntu.

It should be `sudo usermod -aG docker user` instead

## Are there points in the code the reviewer needs to double check?
No

## Does this MR meet the acceptance criteria?

- [x] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)

See merge request !354