Add multiple meta storage in command examples

[anysql]

What would you like to be added:

NAME:
juicefs quota - Manage directory quotas

USAGE:
juicefs quota command [command options] META-URL

DESCRIPTION:

Examples:
$ juicefs quota set redis://localhost --path /dir1 --capacity 1 --inodes 100
$ juicefs quota get redis://localhost --path /dir1
$ juicefs quota list redis://localhost
$ juicefs quota delete redis://localhost --path /dir1

<Please add MySQL/PG/TIKV meta storage example>

.......

Why is this needed:

There is only redis example in the command line help output, and some new users may choose other meta storage than redis, they will ask how to specify a MySQL/PG/TIKV as a meta storage.

[davies]

We don't need to add these in every command, but some where globally.