Doc: use true|false rather than on|off for "failover" option

Doc: use true|false rather than on|off for "failover" option

The CREATE SUBSCRIPTION documentation mentions "false" is the default
option, so let's use true|false rather than on|off in the text which
talks about this option in the ALTER SUBSCRIPTION page.

The other boolean options mentioned in create_subscription.sgml use true
and false so it makes sense to be consistent with these.  The "failover"
option is new to v17.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Ps-RqrggaJU5w85BbeQzw9CLmmLgADVJoJ=xx_4D5CWvw@mail.gmail.com