Update android sdk to 3.3.4 && Fix typos in docs

Author: AbdullahDiaa

README.md

@@ -14,6 +14,9 @@ cordova plugin add pushbots-cordova-plugin --save
 ## Usage
 
 1. Intialize Pushbots in deviceReady section:
+
+**Firebase credentials:** https://www.pushbots.help/en/articles/498201-the-google-part-firebase-credentials
+
 ```javascript
 window.plugins.PushbotsPlugin.initialize("5f4bb916f2e7634ba83f1b93", {"android":{"sender_id":"SENDER_ID", "fcm_app_id":"FCM_APP_ID", "web_api_key":"WEB_API_KEY", "project_id":"PROJECT_ID"}});