reword tuples introduction (#42)

book/src/05_ticket_v2/05_nullability.md

@@ -52,7 +52,8 @@ let y = point.1;
 ### Tuples
 
 It's weird say that something is tuple-like when we haven't seen tuples yet!  
-Tuples are Rust primitive types. They group together a fixed number of values with (potentially different) types:
+Tuples are another example of a primitive Rust type.
+They group together a fixed number of values with (potentially different) types:
 
 ```rust
 // Two values, same type