Update vignette for mixed models, section reshape group levels

[strengejacke]

It says

As you can see, the first row is repeated as it corresponds to the same participant (so the random effects are the same).

But I think after updating reshape_grouplevel(), the behavior changed?

https://easystats.github.io/modelbased/articles/estimate_grouplevel.html

@DominiqueMakowski can you take care of this?