libstdc++: Fix typo in views::as_const's operator() [PR109525]

	PR libstdc++/109525

libstdc++-v3/ChangeLog:

	* include/std/ranges (views::_AsConst::operator()): Add
	missing const to constant_range test.
	* testsuite/std/ranges/adaptors/as_const/1.cc (test02):
	Improve formatting.  Adjust expected type of v2.
	(test03): New test.