Update Api.php

Please see last comment on https://github.com/arcturial/clickatell/pull/10 I made a spelling mistake in previous PR. Code work as expected and there is no errors, only problem is the property mapping to the "mo" parameter is currently "mobile_orginated" where the correct spelling  will be "mobile_originated"
Merge pull request #11 from dTHQb/patch-2

Update Api.php