Typo fix.

src/core/List.pm

@@ -187,7 +187,7 @@ my class List does Positional {
 
     method reverse() {
         self.gimme(*);
-        fail 'Cannot reverse and infinite list' if self.infinite;
+        fail 'Cannot reverse an infinite list' if self.infinite;
         my Mu $rev  := nqp::list();
         my Mu $orig := nqp::clone($!items);
         nqp::push($rev, nqp::pop($orig)) while $orig;