doc/rust: add missing fetchfromGitHub to derivation example

Changed files
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doc
languages-frameworks
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doc/languages-frameworks/rust.section.md
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Rust applications are packaged by using the `buildRustPackage` helper from `rustPlatform`:
```nix
-
{ lib, rustPlatform }:
+
{ lib, fetchFromGitHub, rustPlatform }:
rustPlatform.buildRustPackage rec {
pname = "ripgrep";