'2D Viewport To World' example doesn't follow API docs recommendation

## How can Bevy's documentation be improved?

https://bevyengine.org/examples/2d-rendering/2d-viewport-to-world/

This example adds an input handling system like so: `.add_systems(FixedUpdate, controls)`.
It uses the [`FixedUpdate`](https://docs.rs/bevy/latest/bevy/app/struct.FixedUpdate.html) schedule for this. However, the documentation for the [`Update`](https://docs.rs/bevy/latest/bevy/prelude/struct.Update.html) schedule suggests that _it_ should be used for input handling functions instead. Furthermore, the `FixedUpdate` documentation links to `Update` for "examples of systems that _should not_ use this schedule," so this might cause some confusion.

I'm assuming the API documentation is correct but I don't know whether the example is. I suggest one of these changes:
- If the example's choice of schedule for this system is indeed the preferred option, it should explain why it chooses to deviate from the recommended usage of the schedules per the API docs in a code comment.
- If the example's choice of schedule for this system is incorrect or inconsequential, it should follow the recommendations of the API docs for consistency.

Start with the 2D Viewport To World example at the linked URL, then read the linked Update and FixedUpdate API documentation. Determine whether the controls system should use Update or retain FixedUpdate, and make the example and its explanation consistent with that decision.

Written by the indexing model from the issue text.