change environment variable name

[erral]

The documented environment variable name is wrong, the correct one is COOKIEPLONE_REPOSITORY_TAG

see:

cookieplone/cookieplone/settings.py

Line 40 in 5d538f1

REPO_TAG = "COOKIEPLONE_REPOSITORY_TAG"

[stevepiercy]

@erral would you please add a news item 42.documentation?

@ericof @davisagli I granted Write access to the plone/developers Team to this repo, so that they may open branches directly on this repo instead of their forks. I think it was an oversight. If not, then slap me silly with a halibut.

[davisagli]

Thanks!