Fix note about undefined behavior

[randomPoison]

So turns out this example does trigger undefined behavior. It's undefined behavior to use a pointer to read past the end of the object from which the pointer was originally created. So even though we know there's another array past the end of the first one, it's still undefined behavior to read past the end of the array if we created the initial pointer from the first array. You can see this if you run miri in the playground. If we tweak the example to instead take a pointer to the struct as a whole it runs through miri with no errors, since now we're allowed to read the full contents of the struct.

Isn't undefined behavior fun? 🙃