luci-app-statistics: Adjust also default_timespan to 2hour

Adjust also default_timespan option to be 2hour, as that change was
forgotten in 4b49b456. (That seems to have no practical relevance, as
LuCI seems to offer the first item from the period list in any case.)

Fixes: 4b49b456

 luci-app-statistics: set shortest period data/graph to 2 hours

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>