modify readme

git-svn-id: https://plugins.svn.wordpress.org/googlegraph/trunk@1006128 b8457f37-d9ea-0310-8a92-e5e31aec5664
style

git-svn-id: https://plugins.svn.wordpress.org/googlegraph/trunk@1006129 b8457f37-d9ea-0310-8a92-e5e31aec5664