Adjust details in L17 regarding slices.

[OptimisticPeach]

This does four things:

• The code example used the wrong indices, this fixes it.
• The code actually gets compiled to nothing because it has no side-effects, this uses an &mut reference instead.
• Rust slices are defined to have a contiguous layout in memory and hence when you're able to observe it, the compiler guarantees the layout is as such (in this case because it's going through a reference, it'll always be laid out as described unless it's inlined).
• I dug up a reference I read a few years back about specifically this topic, and added it (although I'm not sure if we're permitted to reference it).

[patricklam]

Looks good to me, thanks! Darn off by one errors. Thanks so much!