fix doctest

Author: untitaker

clippy_lints/src/loops/mod.rs

@@ -73,6 +73,7 @@ declare_clippy_lint! {
     /// ```
     /// Use instead:
     /// ```rust
+    /// let mut arr: [u8; 4] = [1, 2, 3, 4];
     /// arr.copy_within(2..4, 0);
     /// ```
     #[clippy::version = "1.60.0"]