doc: fix example in child_process.md

PR-URL: #13716
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>

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;