Rename `HAIR` (High-level Abstruct IR) to `THIR` (Typed HIR) in this guide

[JohnTitor]

Follow-up of rust-lang/rust#74582
This proposes to rename HAIR to THIR and it'll be likely accepted.
We have some mention of HAIR here, so the follow-up work will be needed as Niko said.
I'll work on this but in case let's create an issue just as a reminder.

[LeSeulArtichaut]

Oh, no more hair puns. 😕

[mark-i-m]

In the immortal words of @Centril: "It's a HAIR cut"