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 <fedor@indutny.com>

doc/api/child_process.markdown

@@ -378,11 +378,8 @@ Example of checking for failed exec:
     var spawn = require('child_process').spawn,
         child = spawn('bad_command');
 
-    child.stderr.setEncoding('utf8');
-    child.stderr.on('data', function (data) {
-      if (/^execvp\(\)/.test(data)) {
-        console.log('Failed to start child process.');
-      }
+    child.on('error', function (err) {
+      console.log('Failed to start child process.');
     });
 
 Note that if spawn receives an empty options object, it will result in