Fix doc warning in bevy_tasks

[nicopap]

Objective

• bevy_tasks emits warnings under certain conditions

When I run cargo clippy -p bevy_tasks the warning doesn't show up, while if I run it with cargo clippy -p bevy_asset the warning shows up.

Solution

• Fix the warnings.

Longer term solution

We should probably fix CI so that those warnings do not slip through. But that's not the goal of this PR.

[hymm]

the warning above can be fixed by replacing the line with

let temp_result = f.await;
result.borrow_mut().replace(temp_result);

[nicopap]

For posterity, I had a section in the PR description showing a clippy lint related to async code, I removed it because I don't want it to be in the commit message.

the warning above can be fixed by replacing the line with

let temp_result = f.await;
result.borrow_mut().replace(temp_result);

Does indeed. I feel dumb. Thought it meant I shouldn't hold result beyond the .await.