Fix time of impact description in ShapeHits

[Jondolf]

Objective

As reported by @nezuo on the Bevy Discord, the documentation for the time_of_impact in ShapeHits is inaccurate and contradictory.

How long the shape travelled before the initial hit, i.e. the distance between the origin and the point of intersection.

These are two separate things: "the distance between the origin and the point of intersection" implies that it corresponds to the blue line, but in reality it is the orange line.

Solution

Clarify the doc comment and remove the incorrect part:

The time of impact (TOI), or how long the shape travelled before the initial hit.