Fix doc bug for cgroup cpuacct usage metric
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

strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc