fix(glue/builtins/derivation): fix comment

The magic builder string is called "builtin:fetchurl", not
"builtins:fetchurl"

Change-Id: I0527aa9ba293807c0da7e67c8d7e9d441de81623
Reviewed-on: https://cl.snix.dev/c/snix/+/30541
Autosubmit: Florian Klink <flokli@flokli.de>
Reviewed-by: Vova Kryachko <v.kryachko@gmail.com>
Tested-by: besadii
Reviewed-by: Ryan Lahfa <ryan@lahfa.xyz>

snix/glue/src/builtins/derivation.rs

@@ -505,7 +505,7 @@ pub(crate) mod derivation_builtins {
                 ))),
         )));
 
-        // If the derivation is a fake derivation (builtins:fetchurl),
+        // If the derivation is a fake derivation (builtin:fetchurl),
         // synthesize a [Fetch] and add it there, too.
         if drv.builder == "builtin:fetchurl" {
             let (name, fetch) =