Remove mentions of "data-averaged posterior" from visualisations vignette

[martinmodrak]

The visualisations vignette (https://hyunjimoon.github.io/SBC/articles/rank_visualizations.html) discusses the examples as if SBC was comparing prior and data-avereged posterior. As I learned since writing that, this is not the correct framing of SBC, so the vignette should be updated to reflect this.

[martinmodrak]

Resolved in 2571083