small text correction in readme file

git-svn-id: https://plugins.svn.wordpress.org/adspace-platform/trunk@767343 b8457f37-d9ea-0310-8a92-e5e31aec5664
small change in readme.txt file

git-svn-id: https://plugins.svn.wordpress.org/adspace-platform/trunk@767344 b8457f37-d9ea-0310-8a92-e5e31aec5664