Improve debug checks (prelude to STL Hardening)

stl/inc/__msvc_ranges_tuple_formatter.hpp

@@ -324,7 +324,7 @@ class basic_format_arg {
         case _Basic_format_arg_type::_Custom_type:
             return _STD forward<_Visitor>(_Vis)(_Custom_state);
         default:
-            _STL_VERIFY(false, "basic_format_arg is in impossible state");
+            _STL_VERIFY(false, "basic_format_arg is in an impossible state");
             int _Dummy{};
             return _STD forward<_Visitor>(_Vis)(_Dummy);
         }