changed readme.txt

git-svn-id: https://plugins.svn.wordpress.org/easythumbnail-sizes/trunk@1028540 b8457f37-d9ea-0310-8a92-e5e31aec5664
Typo.

git-svn-id: https://plugins.svn.wordpress.org/easythumbnail-sizes/trunk@1028541 b8457f37-d9ea-0310-8a92-e5e31aec5664