Rephrase help message for supported provers.

src/main.ml

@@ -164,7 +164,8 @@ let verify_cmd =
   let prover =
     let all_provers = Prover.list_available () in
     let doc =
-      Fmt.str "Prover to use. Available provers are: %s."
+      Fmt.str
+        "Prover to use. Support is provided for the following provers: %s."
         (Fmt.str "%a"
            (Fmt.list ~sep:Fmt.comma Fmt.string)
            (List.map ~f:Prover.to_string all_provers))