Fix bug in README.md
Fix bug in README.md

superagent changed their interface when they went to 1.0
j3parker/superagent-d2l-session-auth,Brightspace/superagent-d2l-session-auth,omsmith/superagent-d2l-session-auth,mdedys/superagent-d2l-session-auth,hekard2l/superagent-d2l-session-auth