README.md: update the version

Author: Sergey Khomushin

README.md

@@ -17,7 +17,7 @@ $ npm install emailjs-com --save
 Or manually: 
 
 ``` html
-<script type='text/javascript' src='https://cdn.jsdelivr.net/npm/emailjs-com@2/dist/email.min.js'></script>
+<script type='text/javascript' src='https://cdn.jsdelivr.net/npm/emailjs-com@3/dist/email.min.js'></script>
 <script type='text/javascript'>
    (function(){
       emailjs.init('<YOUR USER ID>');