Update Slicing subsection of Vector section

Slices can be mutable as well as immutable, contrary to what the text currently says. This commit updates the text, and the exercise, to reflect this.

Author: mckzm

en/src/collections/vector.md

@@ -111,9 +111,9 @@ fn main() {
 
 
 ### Slicing
-A Vec can be mutable. On the other hand, slices are read-only objects. To get a slice, use `&`. 
+Immutable or mutable slices of Vecs can be taken, using `&` or `&mut, respectively.
 
-In Rust, it’s more common to pass slices as arguments rather than vectors when you just want to provide read access. The same goes for `String` and `&str`.
+In Rust, it’s more common to pass immutable slices as arguments rather than vectors when you just want to provide read access. The same goes for `String` and `&str`.
 
 5. 🌟🌟
 ```rust,editable
@@ -129,14 +129,16 @@ fn main() {
     
     assert_eq!(slice1, slice2);
     
-    // Slices are read only
+    // Slices can also be mutable, in which
+    // case mutating them will mutate the 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..3];
-    slice3.push(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!");
 }