Use Printtyp.Compat.signature after Format_doc change

[punchagan]

ocaml/ocaml#13169 added a document type for error messages and this change makes base compatible with OCaml trunk.

I'm not sure how base compatibility with OCaml versions is managed, but I just wanted to notify the team an upcoming breakage (with the next release of OCaml) that I noticed via the Sandmark benchmark builds.