clarify what's in the output

Co-authored-by: Silvan Mosberger <[email protected]>

source/tutorials/callpackage.md

@@ -55,7 +55,7 @@ writeShellScriptBin "hello" ''
 :::{dropdown} Detailed explanation
 `hello.nix` declares a function which takes as argument an attribute set with one element `writeShellScriptBin`.
 [`writeShellScriptBin`](https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeShellScriptBin) is a function that happens to exist in Nixpkgs, a [build helper](https://nixos.org/manual/nixpkgs/unstable/#part-builders) that returns a derivation.
-The derivation output in this case is an executable shell script with the contents `echo "hello world"` named `"hello"`.
+The derivation output in this case contains an executable shell script in `$out/bin/hello` that prints "Hello world" when run.
 :::
 
 Now create a file `default.nix` with the following contents: