Add an example of completely slicing an object.

Author: sw17ch

src/doc/trpl/primitive-types.md

@@ -168,6 +168,7 @@ like arrays:
 ```rust
 let a = [0, 1, 2, 3, 4];
 let middle = &a[1..4]; // A slice of a: just the elements 1, 2, and 3
+let complete = &a[..]; // A slice containing all of the elements in a
 ```
 
 Slices have type `&[T]`. We’ll talk about that `T` when we cover