doc: correct check for failed child_process.spawn
doc: correct check for failed child_process.spawn

Since 0.10.x, exec failures are no longer signaled through stderr,
but rather as error events.

Signed-off-by: Fedor Indutny <e39d94bae70283f13d24c278e04952d6dad9384b@indutny.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node