Update linux_dependencies.md (bevyengine#6205)

for nix build, pkgconfig has been renamed to pkg-config. Very small fix :>

# Objective

- Describe the objective or issue this PR addresses.
- If you're fixing a specific issue, say "Fixes #X".

## Solution

- Describe the solution used to achieve the objective above.

---

## Changelog

> This section is optional. If this was a trivial fix, or has no externally-visible impact, you can delete this section.

- What changed as a result of this PR?
- If applicable, organize changes under "Added", "Changed", or "Fixed" sub-headings
- Stick to one or two sentences. If more detail is needed for a particular change, consider adding it to the "Solution" section
  - If you can't summarize the work, your change may be unreasonably large / unrelated. Consider splitting your PR to make it easier to review and merge!

## Migration Guide

> This section is optional. If there are no breaking changes, you can delete this section.

- If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
- Simply adding new functionality is not a breaking change.
- Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.

docs/linux_dependencies.md

@@ -109,7 +109,7 @@ Add a `shell.nix` file to the root of the project containing:
 { pkgs ? import <nixpkgs> {} }:
 with pkgs; mkShell rec {
   nativeBuildInputs = [
-    pkgconfig
+    pkg-config
     llvmPackages.bintools # To use lld linker
   ];
   buildInputs = [