Remove dead code in exec.xml example

Author: kamil-tekiela

reference/exec/functions/exec.xml

@@ -125,10 +125,8 @@
 <?php
 // outputs the username that owns the running php/httpd process
 // (on a system with the "whoami" executable in the path)
-$output=null;
-$retval=null;
-exec('whoami', $output, $retval);
-echo "Returned with status $retval and output:\n";
+exec('whoami', $output, $result_code);
+echo "Returned with status $result_code and output:\n";
 print_r($output);
 ?>
 ]]>