Improve readability in `./source/behavioral/chain_of_responsibility.swift`

`./source/behavioral/chain_of_responsibility.swift` has an opportunity to improve readability. Consider this snippet from the file: ```swift if let next = self.next { return next.withdraw(amount: amount) } ``` That could be rewritten as: ```swift if let next { return next.withdraw(amount: amount) } ``` Though, it requires newer version of the Swift programming language.