Update book/src/introduction.md
> **NOTE** this hosted version of the `defmt` book corresponds to the *unreleased* `defmt` v0.2.0,
> which is currently only available as a git dependency. If you are looking for `defmt` v0.1.x's
-> documentation head to [https://defmt.ferrous-systems.com](https://defmt.ferrous-systems.com)
+> documentation head to <https://defmt.ferrous-systems.com>.
## Features
