pg_upgrade docs: clarify instructions on standby extensions

pg_upgrade docs:  clarify instructions on standby extensions

Previously the pg_upgrade standby upgrade instructions said not to
execute pgcrypto.sql, but it should have referenced the extension
command "CREATE EXTENSION pgcrypto".  This patch makes that doc change.

Reported-by: a private bug report

Backpatch-through: 9.4, where standby instructions were added