Fix broken links for mysql

[masonbrothers]

The link http://dev.mysql.com/doc/en/adding-users.html appears to be broken.

I think the correct link should be https://dev.mysql.com/doc/refman/en/creating-accounts.html

[tianon]

Nice catch, thanks! I guess we could also pretty reasonably link to the GRANT docs or to https://dev.mysql.com/doc/refman/8.2/en/privileges-provided.html#priv_all for ALL specifically, but this doc links to both of those eventually if someone digs in (and matches what we linked to previously), so I think we're good. 👍