zsh.prezto: fix path in example for 'pmoduleDirs'

Minor fix in documentation. The example in
`programs.zsh.prezto.pmoduleDirs` is not compatible with shell
environment variables like `$HOME`, must be replaced with nix
compatible variables eg: `home.homeDirectory`.

Fixes #4452

modules/programs/zsh/prezto.nix

@@ -34,7 +34,8 @@ let
       pmoduleDirs = mkOption {
         type = types.listOf types.path;
         default = [ ];
-        example = [ "$HOME/.zprezto-contrib" ];
+        example = literalExpression
+          ''[ "''${config.home.homeDirectory}/.zprezto-contrib" ]'';
         description = "Add additional directories to load prezto modules from.";
       };