[fidldev] fix set append -> update.

Fidldev incorrectly used "append" instead of "update" on a set, causing
it to crash when command line arguments for what to regenerate led to
this code path.

Change-Id: Ica7bd632245bcf8b0945334b9dec93b86d037826
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/644882
Reviewed-by: Mitchell Kember <mkember@google.com>