doc: update for haskellPackages.llvmPackages removal

Author: emilazy

doc/frequently-asked-questions.rst

@@ -488,7 +488,7 @@ The solution is to add it as another package to the environment:
 
 .. code:: shell
 
-   $ nix-shell -p '(haskellPackages.ghcWithPackages.override { useLLVM = true; }) (ps: with ps; [mtl])' 'haskellPackages.llvmPackages.llvm'
+   $ nix-shell -p '(haskellPackages.ghcWithPackages.override { useLLVM = true; }) (ps: with ps; [mtl])' 'haskellPackages.ghc.llvmPackages.llvm'
 
    [nix-shell:~]$ llvm-ar --help
    OVERVIEW: LLVM Archiver