docs: fix onRequestError error type example (#94518)
### Fixes #94499
### Summary
The `Instrumentation.onRequestError` type currently exposes the error
parameter as `unknown`, but the docs described it as `Error & { digest:
string }` and the example accessed `err.message` directly.
This updates the docs/example to match the current type and narrows the
caught value before reading `message`.
### Tests:
- `git diff --check`
