fix(primitives-4.rs): update hint so it's less confusing

Author: yamila-moreno

info.toml

@@ -216,7 +216,7 @@ mode = "test"
 hint = """
 Take a look at the Understanding Ownership -> Slices -> Other Slices section of the book:
 https://doc.rust-lang.org/book/ch04-03-slices.html
-and use the starting and ending indices of the items in the Array
+and use the starting and ending (plus one) indices of the items in the Array
 that you want to end up in the slice.
 
 If you're curious why the first argument of `assert_eq!` does not