Change the link to the rendered book in GitHub repo description?

[hcpl]

AIUI, https://docs.rust-embedded.org is now considered the main reference point for embedded docs.

If that's the case, please edit the GitHub repo description link from https://rust-embedded.github.io/book/ to https://docs.rust-embedded.org/book/ for consistency (even if the former one still works).

[hcpl]

Companion issue for Discovery: rust-embedded/discovery#146.