clarify ETo vs ETr in example

[jhuntington]

should make the text of the image example and variable list consistent with what is allowed for reference et usage given that it errors if etr is specified

https://github.com/Open-ET/openet-sims-beta#image-example

[cgmorton]

I have started addressing this in the "tech-team-meeting-updates" branch in Alberto's "openet-sims-beta-1" fork of this repository. Justin, I added you to that repo so you can see if that addresses your issue.