Repair minor error in examples/simple.nix

[c4lliope]

I began learning morph by running morph build examples/simple.nix, and quickly came across an error - the systemdBoot variable had been missing. I made a lucky guess based on other Nix conventions, and had been pleased to see this is only a minor typo.

I hope this helps others learn and run morph hassle-free, because this seems to be the most reliable choice after months of minor annoyances managing colmena.

[srhb]

Hmm, that doesn't seem right. This was just recently added to support specialArgs which really ought to be correct. Did you run morph build example/simple.nix with a morph built from this repo?

Or maybe our impure nixpkgs is confusing here.