docs: fix grammar in Readme (#122)

Author: jmauerhan

README.md

@@ -13,7 +13,7 @@ If you are looking for the SendGrid API client library, please see [this repo](h
 
 # Announcements
 
-All updates to this library is documented in our [CHANGELOG](https://github.com/sendgrid/php-http-client/blob/master/CHANGELOG.md).
+All updates to this library are documented in our [CHANGELOG](https://github.com/sendgrid/php-http-client/blob/master/CHANGELOG.md).
 
 # Table of Contents
 - [Installation](#installation)