Fix Vec::extend_from_slice docs

Author: rukai

library/alloc/src/vec/mod.rs

@@ -2199,7 +2199,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
     /// Clones and appends all elements in a slice to the `Vec`.
     ///
     /// Iterates over the slice `other`, clones each element, and then appends
-    /// it to this `Vec`. The `other` vector is traversed in-order.
+    /// it to this `Vec`. The `other` slice is traversed in-order.
     ///
     /// Note that this function is same as [`extend`] except that it is
     /// specialized to work with slices instead. If and when Rust gets