Minor improvement: lazy evaluation

[Alessandro Candido]

First of all, thanks for your great work!
I'm reviewing many of the things that I learned here and there, finally in a consistent document.

Just one quick suggestion, about the following sentence:

nixos-and-flakes-book/docs/nixos-with-flakes/nixos-with-flakes-enabled.md

Line 188 in 6754096

 By default, a flake will look for a `flake.nix` file in the root directory of each of its dependencies and execute its `outputs` function.  

The meaning is perfectly clear, but, to be precise, Nix is lazy evaluated, so the various output functions are not executed, at least not until the outputs are actually used (and only for those that are used).

I know it is a bit pedantic, but whenever I study something new, I really appreciated simple and straightforward material (and your book is a great example), and even more when it's not sacrificing accuracy at all (because, otherwise, I will pay later the price of taking back something I considered to have learned before).