README: Fix relative path of winit_app.rs module

After originally seeing complaints on `nightly`, we now see them on
`stable` (but no longer on `nightly`): either some regression slipped
in, or `rustc` understands that any relative path references inside
content from `include_str!()` are supposed to be relative to the file
that is being included, not the file that it is being included in
(i.e. in this case relative to `./README.md`, not to `src/lib.rs`
 which includes `../README.md`).

README.md

@@ -73,7 +73,7 @@ use winit::event::{Event, WindowEvent};
 use winit::event_loop::{ControlFlow, EventLoop};
 use winit::window::Window;
 
-#[path = "../examples/utils/winit_app.rs"]
+#[path = "examples/utils/winit_app.rs"]
 mod winit_app;
 
 fn main() {