Clarification: Use Slice instead of Array

There are no arrays in Rust, they are slices. Especially in the tutorial beginners should not be confused with wrong terminology. It helps to know the right names for things when you want to find something in the documentation.

@erickt explained that today to me and it helped me a lot when getting started :wink: Maybe we should also explain what a slice and what a vector is in the tutorial. If you like that, I will try to do that and attach that to the pull request :wink: