Correcting function name in instructions (#1454)

Author: fhightower

exercises/concept/amusement-park/.docs/instructions.md

@@ -42,7 +42,7 @@ attendee.pass_id
 
 ## 4. Revoke the pass
 
-Some guests break the rules with unsafe behavior, so the park wants to be able to revoke passes. Implement `Attendee#revoke_pass` to mutate the state of the instance, and set the pass id to `nil`
+Some guests break the rules with unsafe behavior, so the park wants to be able to revoke passes. Implement `Attendee#revoke_pass!` to mutate the state of the instance, and set the pass id to `nil`
 
 ```ruby
 attendee = Attendee.new(106)