Resolve some issues with CG chapter

[eAlasdair]

Addresses some of the issues raised in #1197

Changes

In section 12.2 Before the block interactives it would be good to explicitly say what directions negative and positive numbers will move the box in.

• Add, in list form

The content mentions point (3, 1) as the rightmost corner of the shape. This refers to the original arrow. The rightmost points of the current shape are (2, 3) and (2, 1)

• Change to the upper rightmost point, (2, 3) and fix relevant calculations