doc-src/Tutorial/Datatype/mutnested

removed special treatment of "_" in syntax (now covered by \<index> arg);

datatype
    'a aexp = IF ('a bexp) ('a aexp) ('a aexp)
            | Sum ('a aexp list)
            | Diff ('a aexp) ('a aexp)
            | Var 'a
            | Num nat
and 'a bexp = Less ('a aexp) ('a aexp)
            | And ('a bexp) ('a bexp)
            | Neg ('a bexp)