[template.mask.array.overview] Fix nesting of parentheses; reflow source

Author: tkoeppe

source/numerics.tex

@@ -8311,18 +8311,11 @@
 
 \pnum
 It has reference semantics to a subset of an array specified by a boolean mask.
-Thus, the expression
-\tcode{a[mask] = b;}
+Thus, the expression \tcode{a[mask] = b;}
 has the effect of assigning the elements of
-\tcode{b}
-to the masked
-elements in
-\tcode{a}
-(those for which the corresponding element
-in
-\tcode{mask}
-is
-\tcode{true}.)
+\tcode{b} to the masked elements in \tcode{a}
+(those for which the corresponding element in
+\tcode{mask} is \tcode{true}).
 
 \rSec3[mask.array.assign]{Assignment}