Fixed code typo.

src/advance/unsafe/superpowers.md

@@ -177,7 +177,7 @@ fn main() {
 
     let r = &mut v[..];
 
-    let (a, b) = r.split_at_mut(3);
+    let (a, b) = split_at_mut(r, 3);
 
     assert_eq!(a, &mut [1, 2, 3]);
     assert_eq!(b, &mut [4, 5, 6]);
@@ -219,6 +219,17 @@ fn split_at_mut(slice: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
         )
     }
 }
+
+fn main() {
+    let mut v = vec![1, 2, 3, 4, 5, 6];
+
+    let r = &mut v[..];
+
+    let (a, b) = split_at_mut(r, 3);
+
+    assert_eq!(a, &mut [1, 2, 3]);
+    assert_eq!(b, &mut [4, 5, 6]);
+}
 ```
 
 相比安全实现，这段代码就显得没那么好理解了，甚至于我们还需要像 C 语言那样，通过指针地址的偏移去控制数组的分割。