Environment Variable RELEASE_UPRADER_ALLOW_THIRD_PARTY spelling is wrong

Think this is an old spelling error.

Fixing this might help others.
Error is in DistUpgrade/DistUpgradeController.py:

  # skip mirror check if special environment is set
        # (useful for server admins with internal repos)
        if (self.config.getWithDefault("Sources","AllowThirdParty",False) or
            "RELEASE_UPGRADER_ALLOW_THIRD_PARTY" in os.environ):
            logging.warning("mirror check skipped, *overriden* via config")
            mirror_check=False