Add trailing slash

README.md

 * [Preventing CSRF Attacks](http://blog.ircmaxell.com/2013/02/preventing-csrf-attacks.html) - An article on preventing CSRF attacks.
 * [Don't Worry About BREACH](http://blog.ircmaxell.com/2013/08/dont-worry-about-breach.html) - An article about the BREACH hack and CSRF tokens.
 * [On PHP 5.3, Lambda Functions and Closures](http://fabien.potencier.org/on-php-5-3-lambda-functions-and-closures.html) - An article about lambda functions and closures.
-* [Use Env](http://seancoates.com/blogs/use-env) - An article about using the unix environment helper.
+* [Use Env](http://seancoates.com/blogs/use-env/) - An article about using the unix environment helper.
 * [Composer Primer](http://daylerees.com/composer-primer/) - A Composer primer.
 * [Composer Versioning](https://igor.io/2013/01/07/composer-versioning.html) - An article about Composer versioning.
 * [Composer Stability Flags](https://igor.io/2013/02/07/composer-stability-flags.html) - An article about Composer stability flags.