Fix typo in comment

pkgs/top-level/default.nix

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   # nixpkgs configuration), infer the other one and platform as needed.
   localSystem = lib.systems.elaborate (
     # Allow setting the platform in the config file. This take precedence over
-    # the inferred platform, but not over an explicitly passed-in onw.
+    # the inferred platform, but not over an explicitly passed-in one.
     builtins.intersectAttrs { platform = null; } config
     // args.localSystem);