Update README to have correct asset URL variable name.
Update README to have correct asset URL variable name.

KazW/heroku-instant-deploy,locomotivecms/heroku-instant-deploy,KazW/heroku-instant-deploy,KazW/heroku-instant-deploy,locomotivecms/heroku-instant-deploy,locomotivecms/heroku-instant-deploy,SuperSchek/schekman-control,SuperSchek/schekman-control,SuperSchek/schekman-control