`Combine` trait example is out of date

[BD103]

In Bevy 0.17 if one of the systems in a system combinator could not be run, the combinator would exit early (using ?). This was changed in #20671 to assume invalid systems returned false, and continue with the combinator logic (using unwrap_or(false)):

bevy/crates/bevy_ecs/src/schedule/condition.rs

Line 1246 in e457025

Ok(a(input, data).unwrap_or(false) ^ b(input, data).unwrap_or(false))

The example for the Combine trait does not reflect this, though:

bevy/crates/bevy_ecs/src/system/combinator.rs

Line 46 in e457025

/// Ok(a((), data)? ^ b((), data)?)

This example should be updated to use unwrap_or(false):

Ok(a((), data).unwrap_or(false) ^ b((), data).unwrap_or(false))