It looks like there's something in the sync.sh script that cron doesn't like.
You could check your email on the box to (or root's) to see if it sent you an email.
Try using bash instead of sh; its more capable. Put it at the top of the script thus '#!/bin/bash' and drop the external call.
Add 'set -xv' next at the top of the script; it'll show you what the parser sees/does & then ensure you capture stdout+stderr to a file to check results.
/usr/share/nginx/html IN_ALL_EVENTS /root/scripts/sync.sh >/tmp/sync.log 2>&1