doc: fix example in child_process.md

PR-URL: https://github.com/nodejs/node/pull/13716
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/child_process.md

@@ -408,7 +408,7 @@ grep.on('close', (code) => {
 ```
 
 
-Example of checking for failed exec:
+Example of checking for failed `spawn`:
 
 ```js
 const spawn = require('child_process').spawn;