Fix description of PostgreSQL::Test::Cluster::wait_for_event()

The arguments of the function were listed in an incorrect order in the
description of the routine.  This information can be seen with perldoc.

Issue spotted while working on this area of the code.

Backpatch-through: 17

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2961,7 +2961,7 @@ sub check_extension
 
 =pod
 
-=item $node->wait_for_event(wait_event_name, backend_type)
+=item $node->wait_for_event(backend_type, wait_event_name)
 
 Poll pg_stat_activity until backend_type reaches wait_event_name.