OpenAIService.close method declared with parentheses

[phelps-sg]

fixes #22

[peterbanda]

OK, to be consistent with client.close() it makes sense. Although on the caller's side having no parentheses is perfectly fine.

[phelps-sg]

This doesn't affect the functionality. However, by convention in Scala declaring a method with no parentheses implies that it has no side effects, ie that it is a pure function. This is misleading in the case of close, because it has side effects on the underlying web socket. In general, we only use parenless methods to retrieve values in such a way that we do not mutate state, as per the example from the Scala guide quoted in #22:

// doesn't change state, call as birthdate
def birthdate = firstName

// updates our internal state, call as age()
def age() = {
  _age = updateAge(birthdate)
  _age
}

Again, the functionality is not affected but it can cause confusion to people reading the code, and confusion can lead to bugs ;-).