Minor Errata with Chapter 12

[yingw787]

I was getting an issue after completing the initial function: Error: Unbound Symbol '\'. I looked at the source code and I realized I needed to add lenv_add_builtin(e, "\\", builtin_lambda) to lenv_add_builtins and that it wasn't being called anywhere else. Everything else works 🐼 💯

[orangeduck]

Thanks! I've updated that chapter.