Update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/encrypt-email/trunk@913531 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.txt to correct a spelling mistake

git-svn-id: https://plugins.svn.wordpress.org/encrypt-email/trunk@913537 b8457f37-d9ea-0310-8a92-e5e31aec5664