Fix unused error

Author: vmcj

webapp/src/Twig/TwigExtension.php

@@ -1162,7 +1162,7 @@ public function problemBadge(ContestProblem $problem, bool $grayedOut = false):
             $rgb = Utils::convertToHex('whitesmoke');
         }
 
-        [$foreground, $border] = $this->hexToForegroundAndBorder($rgb);
+        [$foreground, $border] = Utils::hexToForegroundAndBorder($rgb);
 
         if ($grayedOut) {
             $foreground = 'silver';