[mask.array.comp.assign] Clarify "mask object" (#4096)

Use "mask_array object" instead.

Author: CaseyCarter

source/numerics.tex

@@ -8744,7 +8744,7 @@
 indicated operation to the elements of the argument array and selected elements
 of the
 \tcode{valarray<T>}
-object to which the mask object refers.
+object to which the \tcode{mask_array} object refers.
 \end{itemdescr}
 
 \rSec3[mask.array.fill]{Fill function}