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>

doc/api/child_process.md

@@ -430,7 +430,7 @@ const spawn = require('child_process').spawn;
 
 const child = spawn(process.argv[0], ['child_program.js'], {
   detached: true,
-  stdio: ['ignore']
+  stdio: 'ignore'
 });
 
 child.unref();