Fixes usage of predefined vectors in shooter tutorial

The shooter tutorial references `vec_0` and `vec_x`, which got renamed
to `xy_0` and `xy_x` with improved vector types in f6716aa9. This
updates the usage of those predefined vectors in the turorial code
examples and explanations.
Merge pull request #14 from fdanielsen/fix-vec-usage-shooter-tutorial

Fixes usage of predefined vectors in shooter tutorial