fix syntax of adverb in where clause

This fails after jnthn++'s work on the EXPR preclimit
got things more right.

integration/advent2012-day04.t

@@ -67,7 +67,7 @@ is-deeply @triplet-prods, [31875000], 'Pythagorean triplet products (dataflow)';
 
 my %cache = 1 => 0;
 
-multi factors($n where %cache{$n}:exists) { %cache{$n} }
+multi factors($n where { %cache{$n}:exists }) { %cache{$n} }
 multi factors($n) {
     for 2, 3, *+2 ...^ * > sqrt($n) {
         if $n %% $_ {