Fixed invalid import in documentation code

Author: izagawd

library/core/src/any.rs

@@ -971,7 +971,7 @@ pub const fn try_as_dyn<
 /// ```rust
 /// #![feature(try_as_dyn)]
 ///
-/// use core::any::try_as_dyn;
+/// use core::any::try_as_dyn_mut;
 ///
 /// trait Animal {
 ///     fn speak(&self) -> &'static str;