Fix typo in 2349-pin.md

Author: sinkuu

text/2349-pin.md

@@ -231,7 +231,7 @@ impl<T: ?Sized> PinBox<T> {
 ```
 
 These APIs make `PinBox` a reasonable way of handling data which does not
-implement `!Unpin`. Once you heap allocate that data inside of a `PinBox`, you
+implement `Unpin`. Once you heap allocate that data inside of a `PinBox`, you
 know that it will never change address again, and you can hand out `Pin`
 references to that data.