fix the list of pub unsafe fns

Author: patricklam

doc/src/challenges/new-challenge.md

@@ -45,7 +45,7 @@ All the following pub unsafe functions must be annotated with safety contracts a
 |  Rc<T:?Sized,A:Allocator>::increment_strong_count_in  | alloc::rc |
 |  Rc<T:?Sized,A:Allocator>::decrement_strong_count_in  | alloc::rc |
 |  Rc<T:?Sized,A:Allocator>::get_mut_unchecked | alloc::rc | 
-|  Rc<A:Allocator>::downcast_unchecked | alloc::rc |
+|  Rc<dyn Any,A:Allocator>::downcast_unchecked | alloc::rc |
 |  Weak<T:?Sized>::from_raw | alloc::rc |
 |  Weak<T:?Sized,A:Allocator>::from_raw_in | alloc::rc |
 |  Arc<mem::MaybeUninit<T>,A>::assume_init   |  alloc::sync    |
@@ -55,9 +55,9 @@ All the following pub unsafe functions must be annotated with safety contracts a
 |  Arc<T:?Sized>::decrement_strong_count  | alloc::sync |
 |  Arc<T:?Sized,A:Allocator>::from_raw_in  | alloc::sync |
 |  Arc<T:?Sized,A:Allocator>::increment_strong_count_in  | alloc::sync |
-|  Arc<T:?Sized>::decrement_strong_count_in  | alloc::sync |
-|  Arc<T:?Sized>::get_mut_unchecked | alloc::sync | 
-|  Arc<T:?Sized>::downcast_unchecked | alloc::sync |
+|  Arc<T:?Sized,A:Allocator>::decrement_strong_count_in  | alloc::sync |
+|  Arc<T:?Sized,A:Allocator>::get_mut_unchecked | alloc::sync | 
+|  Arc<dyn Any+Send+Sync,A:Allocator>::downcast_unchecked | alloc::sync |
 |  Weak<T:?Sized>::from_raw | alloc::sync |
 |  Weak<T:?Sized,A:Allocator>::from_raw_in | alloc::sync |