Update sync.rs

Author: Dylan-DPC

library/alloc/src/sync.rs

@@ -1393,7 +1393,7 @@ impl<T: Clone> Arc<T> {
     /// referred to as clone-on-write.
     ///
     /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
-    /// pointers, then the [`Weak`] pointers will be disociated and the inner value will not
+    /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
     /// be cloned.
     ///
     /// See also [`get_mut`], which will fail rather than cloning the inner value