docs: add missing `apps` folder in `prune` command reference (#9658)
### Description
Hi everyone! I found that an example in the `prune` command reference
shows an incorrect folder structure: `frontend` workspace is initially
inside `apps` folder, but it's shown inside `packages` folder in the
output example of `docker prune fronted --docker`
### Screenshots
After running `turbo prune docs --docker`, (think of `docs` as an
equivalent of `frontend`) I get the following structure:

### Testing Instructions
To reproduce it, I created a new turborepo project, and the run `turbo
prune docs --docker`.