MailHandler: Use array access to compare first character

src/Monolog/Handler/MailHandler.php

@@ -70,7 +70,7 @@ abstract class MailHandler extends AbstractProcessingHandler
 
     protected function isHtmlBody(string $body): bool
     {
-        return substr($body, 0, 1) === '<';
+        return $body[0] === '<';
     }
 
     /**