libstdc++: Change comment on #endif to match #if condition [PR124363]

I changed the #if in r8-3123-gc6888c62577671 but didn't make the
corresponding change to the #endif.

libstdc++-v3/ChangeLog:

	PR libstdc++/124363
	* include/std/string_view: Adjust comment on #endif to match #if
	condition.