Fix warning 'config option "threshold_xxx" not found'

The original fix prepended 'threshold_' to $t_threshold, which
is incorrect (maybe a copy/paste error ?).

The 'threshold_' prefix is used to determine the name of the
corresponding GPC variable's name; $t_threshold already contains the
correct config option's name.

Fixes #19970, #16771

manage_config_workflow_set.php

@@ -90,7 +90,7 @@ function config_get_parent( $p_project, $p_option ) {
 	if( $t_access >= $t_access_current ) {
 		$f_value = gpc_get( 'threshold_' . $t_threshold );
 		$t_value_current = config_get( $t_threshold );
-		$t_value_parent = config_get_parent( $t_project, 'threshold_' . $t_threshold );
+		$t_value_parent = config_get_parent( $t_project, $t_threshold );
 		$f_access = gpc_get( 'access_' . $t_threshold );
 		if( $f_value == $t_value_parent && $f_access == $t_access_current ) {
 			# If new value is equal to parent and access has not changed