update readme.txt description

git-svn-id: https://plugins.svn.wordpress.org/captcha-bank/trunk@994447 b8457f37-d9ea-0310-8a92-e5e31aec5664
fixed readme.txt

git-svn-id: https://plugins.svn.wordpress.org/captcha-bank/trunk@994448 b8457f37-d9ea-0310-8a92-e5e31aec5664