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

When implementing this reviewer request:

  #161158 (comment)

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 <sternenseemann@systemli.org>

pkgs/stdenv/linux/make-bootstrap-tools-cross.nix

@@ -17,8 +17,8 @@ in lib.mapAttrs (n: make) (with lib.systems.examples; {
   armv6l-musl  = muslpi;
   aarch64-musl = aarch64-multiplatform-musl;
   riscv64 = riscv64;
-  mips64el-linux-gnuabin32 = lib.systems.platforms.mips64el-linux-gnuabin32;
-  mips64el-linux-gnuabi64  = lib.systems.platforms.mips64el-linux-gnuabi64;
+  mips64el-linux-gnuabin32 = mips64el-linux-gnuabin32;
+  mips64el-linux-gnuabi64  = mips64el-linux-gnuabi64;
   powerpc64 = ppc64;
   powerpc64-musl = ppc64-musl;
   powerpc64le = powernv;