Correction in doc of failover ready steps

Hi,

We have a query in failover-ready doc referring to
pg_subscription_rel. Unlike pg_subscription, pg_subscription_rel gives
results only when connected to the database having the
subscription(s). If we run the concerned query on any other database,
it will give incomplete results i.e. it will give info on main slots
leaving table sync slots (if any).
Thus the failover-ready steps which queries pg_subscription_rel need
to mention that the concerned query needs to be run on the database(s)
that includes the failover enabled subscription(s). Corrected the doc
for the same.

thanks
Shveta