Be more precise about array offset in type layouts

Currently it states that the nth element has an offset of n*(size of type). I think this is assuming that people are starting at 0, but that's not really what `nth` implies. The first element has an offset of 0, but currently it implies that `n=1`, giving it a non-zero offset.

This might want a larger rewording, I added the zero-case explicitly to try and be clearer.

Author: owengage

src/type-layout.md

@@ -71,8 +71,9 @@ at least equal to the size and alignment of a pointer.
 ## Array Layout
 
 Arrays are laid out so that the `nth` element of the array is offset from the
-start of the array by `n * the size of the type` bytes. An array of `[T; n]`
-has a size of `size_of::<T>() * n` and the same alignment of `T`.
+start of the array by `(n-1) * the size of the type` bytes i.e. the `1st` 
+element has an offset of 0. An array of `[T; n]` has a size of 
+`size_of::<T>() * n` and the same alignment of `T`.
 
 ## Slice Layout