Remove duplicated word "only" (#667)

Author: m-housni

README.md

@@ -36,7 +36,7 @@ a GCM API key and VAPID keys.
 ```javascript
 const webpush = require('web-push');
 
-// VAPID keys should only be generated only once.
+// VAPID keys should be generated only once.
 const vapidKeys = webpush.generateVAPIDKeys();
 
 webpush.setGCMAPIKey('<Your GCM API Key Here>');