Fix doc examples to use an exit code

src/lib.rs

@@ -67,7 +67,9 @@
 //!             ..
 //!         } => {
 //!             println!("The close button was pressed; stopping");
-//!             *control_flow = ControlFlow::Exit
+//!             // The number contained by the exit variant determines which exit code the
+//!             // application will have where that's sensible.
+//!             *control_flow = ControlFlow::Exit(0);
 //!         },
 //!         Event::MainEventsCleared => {
 //!             // Application update code.

src/window.rs

@@ -32,7 +32,7 @@ pub use crate::icon::{BadIcon, Icon};
 ///         Event::WindowEvent {
 ///             event: WindowEvent::CloseRequested,
 ///             ..
-///         } => *control_flow = ControlFlow::Exit,
+///         } => *control_flow = ControlFlow::Exit(0),
 ///         _ => (),
 ///     }
 /// });