Bug: Incorrectly added any-pattern into signature of functor

[nikolaushuber]

Describe the bug
ocamlformat seems to insert an any-pattern into the signature of a functor, which results in a syntax error.

How to Reproduce
The following code is accepted by the OCaml compiler:

module type X = sig
  val x : int
end

module M : X -> X =
  functor (X : X) -> struct
    let x = X.x
end 

Following is the output of running ocamlformat on the above code (both if called directly or through dune fmt):

module type X = sig
  val x : int
end

module M : (_ : X) -> X =
functor
  (X : X)
  ->
  struct
    let x = X.x
  end

This results in the following error when trying to compile with OCaml:

File "example/mini_example.ml", line 5, characters 12-13:
5 | module M : (_ : X) -> X =
                ^
Error: Syntax error

I am using version 0.27.0 of ocamlformat, and 5.2.1 of OCaml.