Update Speech_synthesis.md

used es6 approach acc. to contributing guidelines

snippets/Speech_synthesis.md

@@ -6,11 +6,11 @@ It contains the content the speech service should read and information about how
 To know more - https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesisUtterance
 
 ```
-function speak (message) {
+speak = message => {
   var msg = new SpeechSynthesisUtterance(message)
   var voices = window.speechSynthesis.getVoices()
   msg.voice = voices[0]
   window.speechSynthesis.speak(msg)
  }
-speak('Hello, world')
+speak('Hello, World')
 ```