gigasecond: update instructions
replaced chrono with time crate. Reflect this change in instructions.
# Instructions append
-If you're unsure what operations you can perform on `DateTime<Utc>` take a look at the [chrono crate](https://docs.rs/chrono) which is listed as a dependency in the `Cargo.toml` file for this exercise.
+If you're unsure what operations you can perform on `PrimitiveDateTime` take a look at the [time crate](https://docs.rs/time) which is listed as a dependency in the `Cargo.toml` file for this exercise.
