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.

Changed files
+1 -1
nixos
modules
security
+1 -1
nixos/modules/security/ca.nix
···
security.pki.certificateFiles = lib.mkOption {
type = lib.types.listOf lib.types.path;
default = [ ];
-
example = lib.literalExpression ''[ "''${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt" ]'';
description = ''
A list of files containing trusted root certificates in PEM
format. These are concatenated to form
···
security.pki.certificateFiles = lib.mkOption {
type = lib.types.listOf lib.types.path;
default = [ ];
+
example = lib.literalExpression ''[ "''${pkgs.dn42-cacert}/etc/ssl/certs/dn42-ca.crt" ]'';
description = ''
A list of files containing trusted root certificates in PEM
format. These are concatenated to form