[llvm] [Github] Fix libc docs build (PR #70363)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Thanks for the fix! Sorry for the breakage here. I must've completely missed the `-C` when I was looking at minimizing the target list. :sweat_smile: 

https://github.com/llvm/llvm-project/pull/70363