Add funny joke to shell hook example

[3Rafal]

Hello,
In Guide 4 I was ordered to:

Run this to see an example shell hook:
nix develop "github:DeterminateSystems/zero-to-nix#hook"
Nix development environments support environment variables as well. Run echo $FUNNY_JOKE to access a (hilarious) value that's available only in the Nix environment.

The problem is that only #example shell sets this variable and it doesn't work in #hook shell.
This PR makes above example work.

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Name	Status	Preview	Comments	Updated
zero-to-nix	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 3, 2023 at 3:18AM (UTC)

[lucperkins]

@3Rafal I'm gonna think on this a bit. I think that the proper solution here may be to restructure that doc. I basically inserted some new content about shell hooks in a way that didn't fit with the rest of the narrative. Mea culpa! If I end up closing this, please know that it wouldn't be because I think this is "wrong" in any way 😄

[3Rafal]

@lucperkins , no worries! I had fun tinkering with this project, so even if it's not merged I still learned a lot. Thanks for your effort! 😊