[simd.mask.nonmembers] Add 'basic_mask' to subclause heading

Fixes NB US 186-300 (C++26 CD).

Author: jensmaurer

source/numerics.tex

@@ -20365,7 +20365,7 @@
 Nothing.
 \end{itemdescr}
 
-\rSec2[simd.mask.nonmembers]{Non-member operations}
+\rSec2[simd.mask.nonmembers]{\tcode{basic_mask} non-member operations}
 
 \rSec3[simd.mask.binary]{\tcode{basic_mask} binary operators}