Default panic message should print Box<dyn Any>

Prior to this patch, the default panic message (resulting from calling
`panic_any(42);` for example), would print the following error message:

```
thread 'main' panicked at 'Box<Any>', ...
```

However, this should be `Box<dyn Any>` instead.

library/std/src/panicking.rs

@@ -193,7 +193,7 @@ fn default_hook(info: &PanicInfo<'_>) {
         Some(s) => *s,
         None => match info.payload().downcast_ref::<String>() {
             Some(s) => &s[..],
-            None => "Box<Any>",
+            None => "Box<dyn Any>",
         },
     };
     let thread = thread_info::current_thread();