Issue #6358: Correct the documentation for system_configure_date_form…

…ats_form().

By @kiamlaluno and @herbdool.

core/modules/system/system.admin.inc

@@ -2462,7 +2462,7 @@ function system_date_time_formats() {
 /**
  * Allow users to add additional date formats.
  *
- * @param string $format
+ * @param array|null $format
  *   (optional) When present, provides the format being edited, otherwise a new
  *   format is being created.
  */