Fixed an error in the readme.txt.

git-svn-id: https://plugins.svn.wordpress.org/wp-spamshield/trunk@947553 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fixed one more error in readme.txt. Hopefully this is the last one.

git-svn-id: https://plugins.svn.wordpress.org/wp-spamshield/trunk@947554 b8457f37-d9ea-0310-8a92-e5e31aec5664