update README.md

[kweisamx]

fix issue #121

reference:
https://github.com/line/line-bot-sdk-go/blob/master/linebot/send_message.go#L25

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[sugyan]

Oops... Thanks!