Fix URL to Heka documentation in README file

This resolves the following issue:

$ curl -i http://hekad.readthedocs.io/en/
HTTP/1.1 404 OK

Change-Id: I322b74a13f355e929a51fd332f7bc68f1346156d

README.rst

@@ -62,7 +62,7 @@ As well as these infrastructure components:
   and their endpoints.
 - `MariaDB and Galera <https://mariadb.com/kb/en/mariadb/galera-cluster/>`__ for
   highly available MySQL databases
-- `Heka <http://hekad.readthedocs.org/en/>`__ A distributed and
+- `Heka <http://hekad.readthedocs.org/>`__ A distributed and
   scalable logging system for openstack services.
 
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