It's enough for one field to be private. Closes mainmatter#69

Author: LukeMathWalker

book/src/03_ticket_v1/05_encapsulation.md

@@ -23,7 +23,7 @@ To enforce stricter rules, we must keep the fields private[^newtype].
 We can then provide public methods to interact with a `Ticket` instance.
 Those public methods will have the responsibility of upholding our invariants (e.g. a title must not be empty).
 
-If all fields are private, it is no longer possible to create a `Ticket` instance directly using the struct
+If at least one field is private it is no longer possible to create a `Ticket` instance directly using the struct
 instantiation syntax:
 
 ```rust