fix parameter name in general example description
fix parameter name in general example description

I suppose it should be `x1` and `xN` instead of `e1` and `eN` but maybe I misunderstand something because I don't see why only an upper approximation is possible.
lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty