Updated sign up link

Author: aydrian

README.md

@@ -1,6 +1,6 @@
 <a href="https://www.sparkpost.com"><img src="https://www.sparkpost.com/sites/default/files/attachments/SparkPost_Logo_2-Color_Gray-Orange_RGB.svg" width="200px"/></a>
 
-[Sign up](https://app.sparkpost.com/sign-up?src=Dev-Website&sfdcid=70160000000pqBb) for a SparkPost account and visit our [Developer Hub](https://developers.sparkpost.com) for even more content.
+[Sign up](https://app.sparkpost.com/join?plan=free-0817?src=Social%20Media&sfdcid=70160000000pqBb&pc=GitHubSignUp&utm_source=github&utm_medium=social-media&utm_campaign=github&utm_content=sign-up) for a SparkPost account and visit our [Developer Hub](https://developers.sparkpost.com) for even more content.
 
 # SparkPost PHP Library