Fix default sort order (#1575)

This appears to have broken page ordering.

Test-information:

Did a quick test on a personal project, clearing cache and reloading
the page fixed the default sort order of the navbar (i.e. it displayed
according to numeric folder prefixes).

system/src/Grav/Common/Page/Pages.php

@@ -1259,7 +1259,7 @@ class Pages
                 if ($col) {
                     if (($sort_flags & SORT_NATURAL) === SORT_NATURAL) {
                         $list = preg_replace_callback('~([0-9]+)\.~', function($number) {
-                            return sprintf('%032d', $number[0]);
+                            return sprintf('%032d.', $number[0]);
                         }, $list);
                     }