Fix typo in plug-in

contrib/logging/fluentd-es-image/Dockerfile

@@ -27,7 +27,7 @@ RUN /usr/bin/curl -L http://toolbelt.treasuredata.com/sh/install-ubuntu-trusty-t
 # Needed to allow access to /var/log/docker/... files.
 RUN sed -i -e "s/USER=td-agent/USER=root/" -e "s/GROUP=td-agent/GROUP=root/" /etc/init.d/td-agent
 
-# Install the Elasticsearch Fluentd plug0in.
+# Install the Elasticsearch Fluentd plug-in.
 RUN /usr/sbin/td-agent-gem install fluent-plugin-elasticsearch
 
 # Copy the Fluentd configuration file.