[llvm] [llvm][docs] Correct description of %t lit substitution (PR #164397)

[Cullen Rhodes via llvm-commits]

https://github.com/c-rhodes approved this pull request.

Also LGTM cheers, although looks like the description needs updating to drop the mention it's a file (since it's just a path that becomes a file or directory depending on what you do with it):
> Make it clear in the docs that this is the path of a file.

https://github.com/llvm/llvm-project/pull/164397