Explain Constants right after Variables

[cg-cnu]

Chapter 2 Table of contents feels a little out of order.

Current layout:

Variables
Changing Variables
Basic Types
Basic Type Conversion
Constants
Arrays
...

Feels like Constants right after Changing Variables would make more sense.

Proposed layout:

Variables
Changing Variables
Constants
Basic Types
Basic Type Conversion
Arrays
...

Happy to do a PR if everyone agrees. Thanks!