nixos/pixiecore: fix apiServer example

Add missing http:// scheme. Without it pixiecore logs this and never
contacts the API server:

  [DHCP] Couldn't get bootspec for [REDACTED_MAC_ADDR]: Get "localhost:8080/v1/boot/[REDACTED_MAC_ADDR]": unsupported protocol scheme "localhost"

nixos/modules/services/networking/pixiecore.nix

@@ -82,8 +82,8 @@ in
 
       apiServer = mkOption {
         type = types.str;
-        example = "localhost:8080";
-        description = "host:port to connect to the API. Ignored unless mode is set to 'api'";
+        example = "http://localhost:8080";
+        description = "URI to connect to the API. Ignored unless mode is set to 'api'";
       };
 
       extraArguments = mkOption {