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

doc/src/sgml/ref/alter_subscription.sgml

@@ -69,7 +69,7 @@ ALTER SUBSCRIPTION <replaceable class="parameter">name</replaceable> RENAME TO <
    Commands <command>ALTER SUBSCRIPTION ... REFRESH PUBLICATION</command>,
    <command>ALTER SUBSCRIPTION ... {SET|ADD|DROP} PUBLICATION ...</command>
    with <literal>refresh</literal> option as <literal>true</literal> and
-   <command>ALTER SUBSCRIPTION ... SET (failover = on|off)</command>
+   <command>ALTER SUBSCRIPTION ... SET (failover = true|false)</command>
    cannot be executed inside a transaction block.
 
    These commands also cannot be executed when the subscription has