[S17] oops, forgot a dash

Let me just dash back and fix that underscore.

S17-concurrency.pod

@@ -326,7 +326,7 @@ C<Promise.sleep> factory is implemented:
     method sleep(Promise:U: $seconds, :$scheduler = $*SCHEDULER) {
         my $p = Promise.new(:$scheduler);
         my $k = $p.keeper;
-        $scheduler.schedule_in({ $k.keep(True) }, $seconds);
+        $scheduler.schedule-in({ $k.keep(True) }, $seconds);
         $p
     }