update CONTRIBUTING.md

makes Environmental Variables a sub topic

Author: thepriefy

CONTRIBUTING.md

@@ -77,7 +77,7 @@ git clone https://github.com/sendgrid/sendgrid-python.git
 cd sendgrid-python
 ```
 
-## Environment Variables
+### Environment Variables
 
 First, get your free SendGrid account [here](https://sendgrid.com/free?source=sendgrid-python).