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

doc/src/sgml/ref/pgupgrade.sgml

@@ -273,7 +273,8 @@ make prefix=/usr/local/pgsql.new install
      into the new cluster, e.g. <filename>pgcrypto.so</filename>,
      whether they are from <filename>contrib</filename>
      or some other source. Do not install the schema definitions, e.g.
-     <filename>pgcrypto.sql</>, because these will be upgraded from the old cluster.
+     <command>CREATE EXTENSION pgcrypto</>, because these will be upgraded
+     from the old cluster.
      Also, any custom full text search files (dictionary, synonym,
      thesaurus, stop words) must also be copied to the new cluster.
     </para>