README: fix typo

Author: proppy

README.md

@@ -4,7 +4,7 @@
 [![Join the chat at https://gitter.im/googlesamples/firebase-arduino](https://badges.gitter.im/googlesamples/firebase-arduino.svg)](https://gitter.im/googlesamples/firebase-arduino?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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-This repo contains a collection of sample and a library that shows how to call [Firebase](https://www.firebase.com/) from the [ESP8266 Arduino core](https://github.com/esp8266/Arduino).
+This repo contains a collection of samples and an Arduino library that show how to call the [Firebase](https://www.firebase.com/) API from the [ESP8266 Arduino core](https://github.com/esp8266/Arduino).
 
 The Arduino library is [under heavy development](https://github.com/googlesamples/firebase-arduino/issues), **experimental**, **unversioned** and its API is **not stable**.