Fix regex

Author: vmcj

webapp/src/Service/ExternalContestSourceService.php

@@ -530,7 +530,7 @@ protected function loadContest(): void
                 try {
                     // The base URL is the URL of the CCS API root.
                     // Proper is '^(.*\/)contests\/.*/', but PC^2 doesn't expose this (yet).
-                    if (preg_match('/^(.*\/)contest(s\/.*)/',
+                    if (preg_match('/^(.*\/)contest(s\/.*)?/',
                             $this->source->getSource(), $matches) === 0) {
                         $this->loadingError = 'Cannot determine base URL. Did you pass a CCS API contest URL?';
                         $this->cachedContestData = null;