Added an example for environment.variable. (#45956)

Changed files
+1
nixos
modules
+1
nixos/modules/config/shells-environment.nix
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environment.variables = mkOption {
default = {};
+
example = { EDITOR = "nvim"; VISUAL = "nvim"; };
description = ''
A set of environment variables used in the global environment.
These variables will be set on shell initialisation (e.g. in /etc/profile).