Clarify Sys.getenv() on Adobe Analytics page

[gilliganondata]

http://www.dartistics.com/api-adobe-analytics.html

Several people have misinterpreted:

key <- Sys.getenv("ADOBE_KEY_OLD")
secret <- Sys.getenv("ADOBE_SECRET_OLD")

They've put their actual key and secret in these and have not understood that this is the code only if these two environment variables have been created and are being used.

Should be easy enough to clarify.