Update example for new interface of MaybeUninit. (#454)

Author: m-ou-se

src/libs/maintaining-std.md

@@ -260,7 +260,7 @@ impl<T> Drop for OptionCell<T> {
         if self.is_init {
             // Safety: `value` is guaranteed to be fully initialized when `is_init` is true.
             // Safety: The cell is being dropped, so it can't be accessed again.
-            drop(unsafe { self.value.read() });
+            unsafe { self.value.assume_init_drop() };
         }
     }
 }