make clone_from_slice consistent with copy_from_slice

What 'this slice' refers to is not intuitive (we're in the docs for Vec).

Author: alexandermerritt

src/libcollections/slice.rs

@@ -848,7 +848,7 @@ impl<T> [T] {
 
     /// Copies the elements from `src` into `self`.
     ///
-    /// The length of this slice must be the same as the slice passed in.
+    /// The length of `src` must be the same as `self`.
     ///
     /// # Panics
     ///