Fix javascript syntax

Author: jharrilim

guides/source/action_cable_overview.md

@@ -266,9 +266,9 @@ WebSocket is opened.
 createConsumer('https://ws.example.com/cable')
 
 // Use a function to dynamically generate the URL
-createConsumer(getWebSocketURL)
+createConsumer(getWebSocketURL())
 
-function getWebSocketURL {
+function getWebSocketURL() {
   const token = localStorage.get('auth-token')
   return `https://ws.example.com/cable?token=${token}`
 }