Fix doc bug for cgroup cpuacct usage metric

This commit fixes a silly doc bug where the field that represents the
total CPU time consumed by all tasks in the same cgroup was mistakenly
reported as "usage" instead of "usage_nanos".

Relates #21029

docs/reference/cluster/nodes-stats.asciidoc

@@ -220,7 +220,7 @@ the operating system:
     The `cpuacct` control group to which the Elasticsearch process
     belongs
 
-`os.cgroup.cpuacct.usage` (Linux only)::
+`os.cgroup.cpuacct.usage_nanos` (Linux only)::
     The total CPU time (in nanoseconds) consumed by all tasks in the
     same cgroup as the Elasticsearch process