Update RotatingFileHandler.php

Use own constant instead of hardcoded string for default dateFormat (const FILE_PER_DAY = 'Y-m-d';)

src/Monolog/Handler/RotatingFileHandler.php

@@ -50,7 +50,7 @@ class RotatingFileHandler extends StreamHandler
         $this->maxFiles = (int) $maxFiles;
         $this->nextRotation = new \DateTimeImmutable('tomorrow');
         $this->filenameFormat = '{filename}-{date}';
-        $this->dateFormat = 'Y-m-d';
+        $this->dateFormat = self::FILE_PER_DAY;
 
         parent::__construct($this->getTimedFilename(), $level, $bubble, $filePermission, $useLocking);
     }