nixos/pki: change `certificateFiles`'s example to DN42 root cert
The original example is not a good example, `pkgs.cacert` does not need to appear in `security.pki.certificateFiles`, instead, `certificateFiles` will eventually override the input of pkgs.cacert to build the final `caBundle`. Replacing the example with DN42 will both remind users that we are now packaging DN42's root certificates and eliminate the confusion in the previous example.
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