Fix args name of t_get for type t = string with orm.
Fix args name of t_get for type t = string with orm.

We were using "val" as arg name, which is an ocaml keyword. So we are now using "value" ...

mirage/dyntype,mirage/orm,mirage/orm,avsm/ocaml-orm-sqlite,avsm/ocaml-orm-sqlite