Fix metric description of 'Cluster.BytesReadDirectThroughput'
Fix metric description of 'Cluster.BytesReadDirectThroughput'

### What changes are proposed in this pull request?
Fix incorrect description of 'Cluster.BytesReadDirectThroughput' in
docs.

### Why are the changes needed?

Description of metric 'Cluster.BytesReadDirectThroughput' in docs is
incorrect.

### Does this PR introduce any user facing changes?

No.

pr-link: Alluxio/alluxio#15866
change-id: cid-1ab77bdfc194664f0271ecce10bc2ce729960c69
wwjiang007/alluxio,wwjiang007/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,Alluxio/alluxio,Alluxio/alluxio,Alluxio/alluxio,Alluxio/alluxio,Alluxio/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,Alluxio/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,Alluxio/alluxio