doc: rephrase text of child_process.execSync()

Rephrases the error thrown by child_process.execSync().

PR-URL: https://github.com/nodejs/node/pull/14953
Fixes: https://github.com/nodejs/node/issues/14944


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>