Remove superfluous variable assignment

exceptions/MDConsoleInvalidHeadingNumber.php

@@ -12,8 +12,7 @@ final class MDConsoleInvalidHeadingNumber extends Exception {
      */
     public function errorMessage() {
         //error message
-        $errorMsg = 'Invalid heading level';
-        return $errorMsg;
+        return 'Invalid heading level';
 
     }
 }