fix readme example for es-handler

I got the error below while testing the Elasticsearch example in the readme.
Changing es-handler to a Var like the text-handler example fixes it.

```
2013-10-17 09:44:27,624 TRACE itsy.core: :extracting-urls
2013-10-17 09:44:27,648 ERROR itsy.core: Exception executing handler
clojure.lang.ArityException: Wrong number of args (1) passed to: user$es-handler
        at clojure.lang.AFn.throwArity(AFn.java:437)
        at clojure.lang.AFn.invoke(AFn.java:39)
        at itsy.core$crawl_page.invoke(core.clj:93)
        at itsy.core$worker_fn$worker_fn_STAR___3713.invoke(core.clj:134)
        at clojure.lang.AFn.run(AFn.java:24)
        at java.lang.Thread.run(Thread.java:724)
```