Documentation Fix: the QVM URL environment variable should be QVM_URL, not QCS_URL

[bramathon]

Pre-Report Checklist

• [x] I am running the latest versions of pyQuil and the Forest SDK
• [x] I checked to make sure that this bug has not already been reported

Issue Description

In the documentation, it is described that the qvm url can be modified by using the QCS_URL environment variable. I believe this should be the QVM_URL environment variable. I've tested this on my system and it does appear that the QVM_URL is in fact used.