Add an example to avoid error on MySQL 8

[miya0001]

We will have an error with MySQL client 8.x like following.

$ wp db export 
mysqldump: Couldn't execute 'SELECT COLUMN_NAME,                       JSON_EXTRACT(HISTOGRAM, '$."number-of-buckets-specified"')                FROM information_schema.COLUMN_STATISTICS                WHERE SCHEMA_NAME = 'wordpress' AND TABLE_NAME = 'wp_commentmeta';': Unknown table 'COLUMN_STATISTICS' in information_schema (1109)
[miya@mac kankou-kushimoto.test (master)]$ wp db export --column-statistics=0 

This pull request will add an example to avoid this error.
Feel free to update my English. 😊

[schlessera]

Thanks for the pull-request, @miya0001 !

However, I think we should rather check the mysqldump version and add the --column-statistics=0 by default to mysqldump version 8+. This will probably break for everyone, so to avoid lots of support requests, just fixing the issue automatically seems preferable to me.

[miya0001]

I am working for it on #105