Follow flake inputs in the README example (#167)

Author: Pandapip1

README.md

@@ -64,7 +64,10 @@ Here's an example of using it in nixos configuration.
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs = { nixpkgs, rust-overlay, ... }: {