"FluentD Buffer Availability" chart description is incorrect

FluentD Buffer Availability chart shows us the usage of the buffer, not availability. We change the description and chart title as "FluentD Buffer Usage" according to the buffer usage chart.

Author: bysnupy

modules/cluster-logging-dashboards-logging.adoc

@@ -64,8 +64,8 @@ Fetch latency typically takes less time than query latency. If fetch latency is
 |FluentD emit count
 |The total number of Fluentd messages per second for the Fluentd default output, and the retry count for the default output.
 
-|FluentD Buffer Availability
-|The percent of the Fluentd buffer that is available for chunks. A full buffer might indicate that Fluentd is not able to process the number of logs received.
+|FluentD Buffer Usage
+|The percent of the Fluentd buffer that is being used for chunks. A full buffer might indicate that Fluentd is not able to process the number of logs received.
 
 |Elastic rx bytes
 |The total number of bytes that Elasticsearch has received from FluentD, the Elasticsearch nodes, and other sources.