Update Service.php

Is this a a typo?  Intended to be used with \OpenCloud\ObjectStore\Service and other service constructors?  Also ambiguity of 'urlType' as used with archiveType is a little confusing.  Use of constants and better naming would make the ability to use service net (internal) endpoints more obvious.
Merge pull request #480 from ficus/patch-1

Update Service.php - constant value a typo?