docs: add missing shell option to execSync

Adds the "shell" option from child_process.exec to
child_process.execSync on the api docs.

Fixes: #3387
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3440