docs: use rolling nixpkgs in flake docs

Author: sandydoo

docs/guides/using-with-flake-parts.md

@@ -26,7 +26,7 @@ Here's an example of a minimal `flake.nix` file that includes `devenv`:
 ```nix
 {
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
+    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling";
     devenv.url = "github:cachix/devenv";
   };
 

docs/guides/using-with-flakes.md

@@ -37,7 +37,7 @@ Here's a minimal `flake.nix` to start you off that includes:
 ```nix
 {
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
+    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling";
     devenv.url = "github:cachix/devenv";
   };
 
@@ -148,7 +148,7 @@ The `flake.nix` file contains multiple `devShells`. For example:
 ```nix
 {
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
+    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling";
     devenv.url = "github:cachix/devenv";
   };