doc: fix detached child stdio example

The example changed by this commit uses ['ignore'] where
'ignore' is intended.

Fixes: https://github.com/nodejs/node/issues/7269
PR-URL: https://github.com/nodejs/node/pull/7540


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>