change text from "Truth" to "Target"

[nickreich]

This has been requested by CDC colleagues and also aligns with generally how we are trying to move our terminology in hubverse work too. Reasoning being that saying something is "true" is a bit strong, whereas often we want to think of is as the "target" of a forecast or prediction.

I see three places in the 2023/2024 FluSight viz where this appears, maybe there are others too?