Update proc-macro note in extern-prelude section

[XAMPPRocky]

The procedural macro examples use extern crate proc_macro; which as of 1.42.0 is no longer required. https://doc.rust-lang.org/reference/procedural-macros.html

[Centril]

extern crate is still required by the language itself as it doesn't put the crate into the extern prelude. What has changed is that Cargo now puts the crate into the extern prelude, but that's not part of the language.

[XAMPPRocky]

TIL, it's a bit annoying since the reference is the first result people find when searching, but not the worst so I guess we can close.

[ehuss]

Yea, I don't really care one way or the other for the examples.

Ideally we could use better user-level documentation for proc-macros (maybe a dedicated book!). TRPL could mention it, though I'm not sure it is super important.

The note on https://doc.rust-lang.org/reference/items/extern-crates.html#extern-prelude needs to be updated, though. It's been on my todo list for a while. I really should file issues instead of jotting notes.