change "goal" to "task" throughout Tour

In productivity contexts, "goals" tend to be large, long-term projects, whereas "tasks" are smaller actions, and I think "tasks" better matches HabitRPG's features. 

"task" is used in Habit's code in many places. The wiki uses "tasks" rather than "goals" throughout, although admittedly that was a fairly recent change. Many players use "task" when describing habits/dailies/to-dos in the Tavern.