update vector/slicing exercise solution w/ mutable slice material

Author: mckzm

solutions/collections/Vector.md

@@ -156,21 +156,24 @@ fn main() {
     let mut v = vec![1, 2, 3];
 
     let slice1 = &v[..];
-    // out of bounds will cause a panic
+    // Out of bounds will cause a panic
     // You must use `v.len` here
-    let slice2 = &v[0..v.len()];
+    let slice2 = &v[0..3];
 
     assert_eq!(slice1, slice2);
 
-    // slice are read only
+    // A slice can also be mutable, in which
+    // case mutating it will also mutate its underlying Vec
     // Note: slice and &Vec are different
     let vec_ref: &mut Vec<i32> = &mut v;
     (*vec_ref).push(4);
-    let slice3 = &mut v[0..];
+    let slice3 = &mut v[0..4];
+    slice3[3] = 42;
 
-    assert_eq!(slice3, &[1, 2, 3, 4]);
+    assert_eq!(slice3, &[1, 2, 3, 42]);
+    assert_eq!(v, &[1, 2, 3, 42]);
 
-    println!("Success!")
+    println!("Success!");
 }
 ```