Spec: Update Index and Slice section with explicit callout to copying composite types

[Rossiar]

I don't think the spec makes the behaviour of copy by reference clear when it says:

A slice is a way to access portions of an array or a string. It is a substring or subarray that is copied from the original array or string.

It would be good to add an example to this section of the spec to highlight that composite types are copied by reference. Something like:

x := [[1 2] [3 4]]
y := x[:]
y[0] = [8 9]
print "x:" x
print "y:" y
---
x: [[8 9] [3 4]]
y: [[8 9] [3 4]]