fix grammar in exercise 06 from section 03 (#248)

Add missing "to" preposition in first TODO comment

exercises/03_ticket_v1/06_ownership/src/lib.rs

@@ -1,6 +1,6 @@
 // TODO: based on what we just learned about ownership, it sounds like immutable references
 //   are a good fit for our accessor methods.
-//   Change the existing implementation of `Ticket`'s accessor methods take a reference
+//   Change the existing implementation of `Ticket`'s accessor methods to take a reference
 //   to `self` as an argument, rather than taking ownership of it.
 
 pub struct Ticket {