adjust doc to properly reflect accepted params

Author: Andrei Motinga

lib/phaxio/resources/ata.rb

@@ -89,6 +89,7 @@ def list params = {}
         #   - *name* [String] - A name used to identify the ATA.
         #   - *description* [String] - A longer description of the ATA.
         #   - *domain* [String] - A domain for the ATA.
+        #   - *mac_address* [String] - A mac address for the ATA.
         # @return [Phaxio::Resources::Ata]
         #   The created ATA, including the generated username and password.
         # @raise [Phaxio::Error::PhaxioError]