prefix fetchTarball with builtins. too

For consistency/clarity.

README.md

@@ -21,7 +21,7 @@ Afterwards, create a `default.nix` file containing the following:
       lock = builtins.fromJSON (builtins.readFile ./flake.lock);
       inherit (lock.nodes.flake-compat.locked) narHash rev url;
     in
-    fetchTarball {
+    builtins.fetchTarball {
       url = "${url}/archive/${rev}.tar.gz";
       sha256 = narHash;
     }