update readme

git-svn-id: https://plugins.svn.wordpress.org/tpg-get-posts/trunk@876813 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme update cat to category

git-svn-id: https://plugins.svn.wordpress.org/tpg-get-posts/trunk@876815 b8457f37-d9ea-0310-8a92-e5e31aec5664