Update README.md (#144)

Fix: Messagetype datatype

Author: s4nk37

README.md

@@ -66,7 +66,7 @@ List<Message> messageList = [
 
 6. Adding a `onSendTap`.
 ```dart
-void onSendTap(String message, ReplyMessage replyMessage, Message messageType){
+void onSendTap(String message, ReplyMessage replyMessage, MessageType messageType){
   final message = Message(
     id: '3',
     message: "How are you",
@@ -511,4 +511,4 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
-```
+```