fix: dma example

Author: Ddystopia

text/0000-forget-marker-trait.md

@@ -188,7 +188,13 @@ Let's say that `Serial::read_exact` triggers a DMA transfer and returns a future
 fn start(serial: &mut Serial) {
     let mut buf = [0; 16];
 
-    mem::forget(serial.read_exact(&mut buf));
+    let fut = Box::pin(serial.read_exact(&mut buf));
+
+    fut.poll(cx); // start dma transfer
+
+    // Memory of the future itself is still valid (inside the allocation),
+    // but the buffer lives outside of it and is not protected.
+    core::mem::forget(fut); // or `Box::leak`
 }
 
 fn corrupted() {