address @reynir review comment

pk/mirage_crypto_pk.mli

@@ -187,11 +187,11 @@ module Rsa : sig
         was produced with the given [key] as per {{!sig_encode}sig_encode}, or
         [None] *)
 
-    val min_key : [ `MD5 | `SHA1 | `SHA224 | `SHA256 | `SHA384 | `SHA512 ] -> bits
+    val min_key : [< Digestif.hash' > `MD5 `SHA1 `SHA224 `SHA256 `SHA384 `SHA512 ] -> bits
     (** [min_key hash] is the minimum key size required by {{!sign}[sign]}. *)
 
     val sign : ?crt_hardening:bool -> ?mask:mask ->
-      hash:[ `MD5 | `SHA1 | `SHA224 | `SHA256 | `SHA384 | `SHA512 ] ->
+      hash:[< Digestif.hash' > `MD5 `SHA1 `SHA224 `SHA256 `SHA384 `SHA512 ] ->
       key:priv -> string or_digest -> string
     (** [sign ~crt_hardening ~mask ~hash ~key message] is the PKCS 1.5
         signature of [message], signed by the [key], using the hash function
@@ -205,8 +205,7 @@ module Rsa : sig
 
         @raise Invalid_argument if message is a [`Digest] of the wrong size.  *)
 
-    val verify :
-      hashp:([ `MD5 | `SHA1 | `SHA224 | `SHA256 | `SHA384 | `SHA512 ] -> bool) ->
+    val verify : hashp:([< Digestif.hash' > `MD5 `SHA1 `SHA224 `SHA256 `SHA384 `SHA512 ] -> bool) ->
       key:pub -> signature:string -> string or_digest -> bool
     (** [verify ~hashp ~key ~signature message] checks that [signature] is the
         PKCS 1.5 signature of the [message] under the given [key].