[llvm] [NFC] Fix typos in llvm-mc doc (PR #72457)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Probably quite a few really minor nits for a small patch. Feel free to ignore them if you disagree with them. Just thought we could try and improve the clarity even a bit more while you're here.

Thanks for taking the time to help improve the documentation!

https://github.com/llvm/llvm-project/pull/72457