Fix onSend type
Fix onSend type

to match https://github.com/FaridSafi/react-native-gifted-chat/blob/656a700106923e7e92a5a4644e0729fef35f0a30/src/GiftedChat.tsx#L674
FaridSafi/react-native-gifted-chat,FaridSafi/react-native-gifted-chat