Fix mismatched FogFalloff

[lomirus]

Objective

When user presses 3, the falloff mode should be changed to ExponentialSquared as described in the instructions, but it's not in fact.

Online Example: https://bevyengine.org/examples-webgpu/3d-rendering/fog/

Solution

Change it to ExponentialSquared

Testing

• Did you test these changes? If so, how?

Yes, by cargo run --example fog

• Are there any parts that need more testing?

No.

• How can other people (reviewers) test your changes? Is there anything specific they need to know?

cargo run --example fog

• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

N/A