use function system in bevy lib.rs docs

src/lib.rs

@@ -14,11 +14,13 @@
 //!fn main() {
 //!    App::build()
 //!        .add_default_plugins()
-//!        .add_system(|_: &mut World, _: &mut Resources| {
-//!            println!("hello world");
-//!        })
+//!        .add_system(hello_world_system.system())
 //!        .run();
 //!}
+//!
+//!fn hello_world_system() {
+//!    println!("hello world");
+//!}
 //! ```
 
 //! Don't let the simplicity of the example above fool you. Bevy is a [fully featured game engine](https://bevyengine.org/learn/book/introduction/features/)