docs: Fix stderr debugging guidance (#13122)
### Description
Fixed the TUI debugging guidance that incorrectly described where
`eprintln!()` writes output.
`eprintln!()` writes to `stderr`, not `stdout`, so this updates the
explanation and example redirection command to match `stderr`. This also
fixes a `sterr` typo in `ARCHITECTURE.md`.
### Testing Instructions
Documentation-only change, so no tests were run.
Verified the changed scope and checked for the remaining typo with:
```bash
rg -n "sterr|eprintln|2&>|stderr|stdout" CONTRIBUTING.md crates/turborepo/ARCHITECTURE.md
git diff -- CONTRIBUTING.md crates/turborepo/ARCHITECTURE.md
