chore: update signal test description to test v0.2.3 (#281)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/spec/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request:  ooni/probe#2627
- [x] related ooni/probe-cli pull request: ooni/probe-cli#1389
- [x] If I changed a spec, I also bumped its version number and/or date

## Description

This updates the signal test spec to the changes to the tested endpoints