Fix paste-o in Arc::make_slice_mut() test.

Author: kpreid

library/alloc/src/sync.rs

@@ -2210,7 +2210,7 @@ impl<T: Clone, A: Allocator + Clone> Arc<[T], A> {
     /// #![feature(make_mut_slice)]
     /// use std::sync::Arc;
     ///
-    /// let mut data: Arc<[i32]> = Rc::new([10, 20, 30]);
+    /// let mut data: Arc<[i32]> = Arc::new([10, 20, 30]);
     ///
     /// Arc::make_mut_slice(&mut data)[0] += 1;        // Won't clone anything
     /// let mut other_data = Arc::clone(&data);        // Won't clone inner data