Rust/Gigasecond mentoring.md outdated

[Fatcat560]

A while back, the Rust exercise for the Gigasecond problem has been updated to use a different dependency (see exercism/rust#1384 ). However it seems that, while most of the important documentation has been updated to reflect the change, the current mentoring.md file still references the old chrono dependency.

From the looks of it, most of the advice still holds true even with a different dependency, however it would be good to have some info about the used crate in this exercise for mentors.

I sadly haven't used the time crate much myself so I'm hoping someone could share some knowledge here so the file can be modified with the best practices for the time crate 😄