Polymorphic field accessor functions don't always pretty-print properly

[mn200]

The following

val _ = Hol_datatype`rcd = <| data : 'a list ; len : num |>`;
``rcd_data : 'b rcd -> 'b list``;

results in

val it = ``<thyname>$rcd_data``

being printed. If rcd_data has the 'a type, then it prints without the (unnecessary) theory-qualification.