Point to the more recent Little Book of Rust Macros

[peter-lyons-kehl]

https://doc.rust-lang.org/nightly/book/ch19-06-macros.html has a link "The Little Book of Rust Macros” to non-mdbook https://danielkeep.github.io/tlborm/book/index.html. That is 5 years old (see https://github.com/DanielKeep/tlborm).

Suggest replacing the link with https://veykril.github.io/tlborm (which is an updated and md-book version of the above).

If approved, I'm willing to provide a PR.

[carols10cents]

This was done in #2722. Thanks!