[DOCS] fix sentence in monitoring.asciidoc
Relates to
=== Monitoring and Security
<<xpack-monitoring, {monitoring}>> consists of two components: an agent
-that you install on on each {es} and Logstash node, and a Monitoring UI
+that you install on each {es} and Logstash node, and a Monitoring UI
in {kib}. The monitoring agent collects and indexes metrics from the nodes
and you visualize the data through the Monitoring dashboards in {kib}. The agent
can index data on the same {es} cluster, or send it to an external
