Nit: Misspelled "null" in docblock typing

Update AuthenticatesCluster.php

src/Traits/Cluster/AuthenticatesCluster.php

@@ -76,7 +76,7 @@ public function withToken(string $token = null)
      * Load the token from provider command line.
      *
      * @param  string  $cmdPath
-     * @param  string|nll  $cmdArgs
+     * @param  string|null  $cmdArgs
      * @param  string|null  $tokenPath
      * @return $this
      */