vector/slicing - improve the phrasing

Author: mckzm

en/src/collections/vector.md

@@ -129,8 +129,8 @@ fn main() {
     
     assert_eq!(slice1, slice2);
     
-    // Slices can also be mutable, in which
-    // case mutating them will mutate the underlying Vec
+    // A slice can also be mutable, in which
+    // case mutating it will mutate its underlying Vec
     // Note: slice and &Vec are different
     let vec_ref: &mut Vec<i32> = &mut v;
     (*vec_ref).push(4);