nixos/misc/nixpkgs: fixed syntax error in overlays example

I was pointed towards a small syntax error in the `nixpkgs.overlays`
documentation. There was a trailing semicolon after the overlay
function.

I also aligned the code a bit better so opening and closing brackets can
be visually matched much better (IMO).

nixos/modules/misc/nixpkgs.nix

@@ -127,13 +127,14 @@ in
       default = [];
       example = literalExample
         ''
-          [ (self: super: {
+          [
+            (self: super: {
               openssh = super.openssh.override {
                 hpnSupport = true;
                 kerberos = self.libkrb5;
               };
-            };
-          ) ]
+            })
+          ]
         '';
       type = types.listOf overlayType;
       description = ''