Renamed the key 'check' into 'health-check'

The key 'check' is already used in meta/heka.yml to enable/disable the
monitoring of the haproxy backends.

Change-Id: I7c82b5a01b1679a99e14818fa5b284b38e1bc52b
diff --git a/README.rst b/README.rst
index 0e9bd9f..7495992 100644
--- a/README.rst
+++ b/README.rst
@@ -261,7 +261,7 @@
       listen:
         redis_cluster:
           service_name: redis
-          check:
+          health-check:
             tcp:
               enabled: True
               options: