[mask.array.assign] Replace "it" with its antecedent (#4095)

Many nouns appear in this sentence before "it", _none_ of which is the proper antecedent.

Author: CaseyCarter

source/numerics.tex

@@ -8718,7 +8718,7 @@
 These assignment operators have reference semantics, assigning the values
 of the argument array elements to selected elements of the
 \tcode{valarray<T>}
-object to which it refers.
+object to which the \tcode{mask_array} object refers.
 \end{itemdescr}
 
 \rSec3[mask.array.comp.assign]{Compound assignment}