docs: makes Environmental Variables a sub topic in CONTRIBUTING.md (#387)

Author: thepriefy

CONTRIBUTING.md

@@ -72,7 +72,7 @@ git clone https://github.com/sendgrid/sendgrid-java.git
 cd sendgrid-java
 ```
 
-## Environment Variables
+### Environment Variables
 
 First, get your free Twilio SendGrid account [here](https://sendgrid.com/free?source=sendgrid-java).