AUTO_RANDOM doc says 'number of storage engines' when it should be number of TiKV nodes

[mjonss]

Error Report

Please answer the following questions before submitting your issue. Thanks!

1. What is the URL/path of the document related to this issue?
   https://docs.pingcap.com/tidb/stable/auto-random

2. How would you like to improve it?
   Change:
   The best practice is to set the shard bits as log(2, x), in which x is the current number of storage engines.
   to
   The best practice is to set the shard bits as log(2, x), in which x is the current number of TiKV nodes.