Add example to --systems help (#482)

Without it, the syntax is unclear. For instance, I tried commas, and that didn't work.

Co-authored-by: Jörg Thalheim <[email protected]>
Co-authored-by: Peder Bergebakken Sundt <[email protected]>

Author: 3 people

nixpkgs_review/cli/__init__.py

@@ -219,7 +219,7 @@ def common_flags() -> list[CommonFlag]:
             "--systems",
             type=str,
             default="current",
-            help="Nix 'systems' to evaluate and build packages for",
+            help="Nix 'systems' to evaluate and build packages for (e.g. 'all' or 'x86_64-linux aarch64-darwin')",
         ),
         CommonFlag(
             "--system",