Update README.md

Author: thinkingserious

README.md

@@ -13,7 +13,7 @@ All updates to this project is documented in our [CHANGELOG](https://github.com/
 ## Prerequisites
 
 - Python version 2.6, 2.7, 3.4 or 3.5
-- The SendGrid service, starting at the [free level](https://sendgrid.com/free?source=sendgrid-python)
+- The SendGrid service, starting at the [free level](https://sendgrid.com/free?source=python-http-client)
 
 ## Install Package