Revise groupBy example when double relationship not needed

[ribiza]

Revise the example

{
  Book(groupBy: [genre]) {
    genre
    author{name}
    MAX(GROUP: { field: AVG })
    GROUP (groupBy: [author]){
      author { name }
      AVG(GROUP: { field: rating })
      GROUP {
        title
        rating
      }
    }
  }
}

in group.md when sourcenetwork/defradb#4952 is fixed.