doc/rust: add missing fetchfromGitHub to derivation example

doc/languages-frameworks/rust.section.md

···
 Rust applications are packaged by using the `buildRustPackage` helper from `rustPlatform`:
 
 ```nix
-{ lib, rustPlatform }:
+{ lib, fetchFromGitHub, rustPlatform }:
 
 rustPlatform.buildRustPackage rec {
   pname = "ripgrep";