Server uptime stats does not show 'week'

In response to the _/face/stats/_ URI, there is a [typo](https://github.com/duckduckgo/cpp-libface/blob/8aa29425f4f8e9982732d3b7b3798ff3f3a3d356/src/main.cpp#L439) in the _humanized_time_difference_ function.
This prevents the response to show the uptime _week_ and shows _day_ instead.

And yeah, _libface_ is amazing. @dhruvbird :zap: 
