Move note about parallelism above examples in The Book > Getting Started > ECS

[PROMETHIA-27]

Can the quick note, ""hello world!" might show up in a different order than it does above. This is because systems run in parallel by default whenever possible." be moved just above the output box at the bottom of The Book/Getting Started/ECS? Someone (and possibly more that did not publicly comment on it) did not see it until it was pointed out and it made the process of learning bevy unnecessarily confusing/frustrating.

It should be a 30 second change, I just don't have the setup to make a PR for this atm.