Fix Variable name

[patricia-ternes]

Later on, the variable is always called as new_pal. So for consistence, I suggest to rename this.

https://github.com/ARCTraining/data-vis/blob/26126081433b8220d41df332cd7a17cb3fefd621/docs/05-composition-bar-charts.md?plain=1#L96C2-L96C13