[c.math.fpclass] Update to refer to C23 7.12.4, 7.12.18

... partially due to new sections added after WG14 N3220, along with
typo fix

Author: frederick-vs-ja

source/numerics.tex

@@ -9949,7 +9949,7 @@
 The classification / comparison functions behave the same as the C macros with the
 corresponding names defined in the C standard library.
 
-\xrefc{7.12.3, 7.12.4}
+\xrefc{7.12.4, 7.12.18}
 
 \rSec2[sf.cmath]{Mathematical special functions}%