[llvm] [Github] fix libc documentation path (PR #118514)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

I'm not sure if this issue is more pervasive than just libc, or if we should perhaps change something about how we build the libc docs (where they wind up in the build dir)?

https://github.com/llvm/llvm-project/pull/118514