libstdc++: Correct changelog of previous commit

After committing it I noticed I'd also accidentally added a change to
__synth3way as well, which I meant to do in a separate commit. I've
updated the changelog entry to reflect that additional change.

	* libsupc++/compare (__detail::__synth3way): Add noexcept-specifier.

libstdc++-v3/ChangeLog

@@ -2,6 +2,7 @@
 
 	* include/std/version (__cpp_lib_three_way_comparison): Update value.
 	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.
+	(__detail::__synth3way): Add noexcept-specifier.
 
 	* include/bits/stl_map.h (map): Define operator<=> and remove
 	operator< for C++20.