Update "Failover with Keepalived" tutorial in KB

The "Failover with Keepalived" tutorial seems to be missing some details that are available in a more recent blog post. For example, compare the following documentation page:

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-failover-with-keepalived/

To the following blog post:

https://mariadb.com/resources/blog/maxscale-ha-setup-using-keepalived-and-maxctrl

A major difference is that the blog post has some great information in the "MaxScale active/passive-setting" section, where it mentions how to use maxctrl to change the active instance.

It looks like this may have already been "Fixed" as part of MXS-1638, but I still don't see the information in the KB.