util-core: Add Scaladoc Example to `Time.at`
util-core: Add Scaladoc Example to `Time.at`

Problem / Solution

GitHub issue #256 was opened due to a misunderstanding of how the
`Time.at` method works. Let's add an example to give a demonstration
of how to use it.

Differential Revision: https://phabricator.twitter.biz/D369452

twitter/util,twitter/util,twitter/util