Add missing trailing newline in multi-line string example

[niklasmohrin]

I was just checking out the Nix language basics and found the example of multi-line strings a bit odd as it exhibits different (and weirder) behavior than Python """ strings and is inconsistent with the second example. Checking with the NixOS Virtualbox image shows that in reality, the behavior is as expected and this seems to be a typo :)

[github-actions]

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[niklasmohrin]

I don't think I have any copyright here due to minority, if possible I would prefer not to "sign" your document. If not, please feel free to reproduce the fix yourself in another PR

[niklasmohrin]

Ah, it seems that there is no CLA anymore - problem solved :)

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/this-month-in-nix-docs-2-april-2023/27899/1