Replace @ in README with deref

Some lein plugins, such as [lein-ancient](https://github.com/xsc/lein-ancient), read profiles.clj as EDN. The @ reader macro is not valid EDN, so I changed the example in the README to use deref instead.

Exact usage: https://github.com/xsc/lein-ancient/blob/master/src/leiningen/ancient/utils/io.clj#L93