Update vec.md (#2810)

With #1898 the slices were moved to day 1. So I deleted the day 3 slice
reference sentence in day 2 vec.md.

src/std-types/vec.md

@@ -49,7 +49,5 @@ methods on a `Vec`.
 - To index the vector you use `[` `]`, but they will panic if out of bounds.
   Alternatively, using `get` will return an `Option`. The `pop` function will
   remove the last element.
-- Slices are covered on day 3. For now, students only need to know that a value
-  of type `Vec` gives access to all of the documented slice methods, too.
 
 </details>