Update error message: #[repr(transparent)] is now stable

[lo48576]

#1 (comment).

Now #[repr(transparent)] is stabilized, so it should be primarily suggested (rather than #[repr(C)]).

opaque_typedef/opaque_typedef_macros/src/type_props/builder.rs

Lines 70 to 79 in a454844

	// Neither `repr(C)` nor `repr(transparent)` was specified for an unsized type.
	panic!(
	"To avoid undefined behavior, outer type `{}` should be marked \
	as `#[repr(C)]` or `#[repr(transparent)]`.\n\
	For detail, see <https://github.com/lo48576/opaque_typedef/issues/1>.\n\
	About `#[repr(transparent)]`, see RFC 1758 \
	<https://github.com/rust-lang/rfcs/blob/master/text/1758-repr-transparent.md>.\
	",
	ty_outer
	);