fixed readme.txt

git-svn-id: https://plugins.svn.wordpress.org/wp-add-mime-types/trunk@756978 b8457f37-d9ea-0310-8a92-e5e31aec5664
fixed version number

git-svn-id: https://plugins.svn.wordpress.org/wp-add-mime-types/trunk@756979 b8457f37-d9ea-0310-8a92-e5e31aec5664