fix field name in error message

public/volunteer.php

@@ -45,7 +45,7 @@ if (!isset($position)) {
     http_response_code(400);
 	header('Content-Type: application/json');
     die(json_encode([
-	    'field' => 'full_name',
+	    'field' => 'position',
 	    'message' => 'Error: A volunteer position is required.'
     ]));
 }