[llvm] Fix GetElementPtr article link (PR #151868)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue commented:

This document is not meant to be viewed on GitHub, the place to go is https://llvm.org/docs/FAQ.html. 

I don't think this is using the correct form of link (see suggestion), but the correct form also won't work on GitHub.

https://github.com/llvm/llvm-project/pull/151868