doc: Use .copied() instead of .cloned() in Vec example

Author: meithecatte

src/liballoc/vec.rs

@@ -92,7 +92,7 @@ use crate::raw_vec::RawVec;
 /// vec[0] = 7;
 /// assert_eq!(vec[0], 7);
 ///
-/// vec.extend([1, 2, 3].iter().cloned());
+/// vec.extend([1, 2, 3].iter().copied());
 ///
 /// for x in &vec {
 ///     println!("{}", x);