[ESS] typo in documentation: essa-r?

[Stephen Eglen]

> "ESS[R]: After (require 'essa-r), M-x ess-r-var allows to load (...)" but
> I think it should be "ess-r-a", not "essa-r".

Thanks Ramon; I think this is historical; at the time the note was
written, the file was called essa-r.el; but last year many files got
renamed for consistency, and this was one of the files.

We could probably do a better job of our history, but I'd rather not
edit these old notes.  

Stephen