make-bootstrap-tools-cross: fix typo which prevents hydra eval
make-bootstrap-tools-cross: fix typo which prevents hydra eval

When implementing this reviewer request:

  https://github.com/NixOS/nixpkgs/pull/161158#discussion_r822256070

to move mips64el-unknown-linux-* from platforms.nix to examples.nix, I
neglected to update the reference in make-bootstrap-tools-cross.nix.
As a result, the hydra jobs to generate a bootstrap tarball for
mips64el are not running:

  https://hydra.nixos.org/eval/1750828?filter=mips&compare=1750620&full=#tabs-errors

This commit fixes the problem, so the hydra job can run.  Once it
does, I will submit a PR adding the trusted bootstrap tarball hash to
pkgs/stdenv/linux/bootstrap-files/.

Co-authored-by: sterni <91fa371d5c043421c57d5f378aac78fdc4ba4dfc@systemli.org>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs