[README.md] Fix typo in OAuth example
[README.md] Fix typo in OAuth example

The success handler's access token is defined as an optional value in the implementation.
```swift
public typealias TokenSuccessHandler = (accessToken: SwifterCredential.OAuthAccessToken?, response: NSURLResponse) -> Void
```
cjwirth/Swifter,brandons/Swifter,ibru/Swifter,SteveClement/Swifter,mattiabugossi/Swifter,oktapodi/Swifter,Gurpartap/Swifter,brandons/Swifter,s-aska/Swifter,kristenm6/Swifter,harlanhaskins/Swifter-1,siuying/Swifter,babyboy18/swift_code,gecko655/Swifter,adarwash2012/Swifter,mattdonnelly/Swifter,mattdonnelly/Swifter,cjwirth/Swifter,harlanhaskins/Swifter-1,simorgh3196/Swifter,Paludis/Swifter