Fix improper example of using psql() function in TAP tests documentation.

In an example of TAP test scripts, there is the test checking whether
the result of the query is expected or not. But, in previous example,
the exit code of psql instead of the query result was checked unexpectedly.

Author: Ildar Musin

src/test/perl/README

@@ -41,7 +41,7 @@ against them and evaluate the results. For example:
     $node->init;
     $node->start;
 
-    my $ret = $node->psql('postgres', 'SELECT 1');
+    my $ret = $node->safe_psql('postgres', 'SELECT 1');
     is($ret, '1', 'SELECT 1 returns 1');
 
     $node->stop('fast');