This is how you do it.
Edit this file;
Find the line starting with
#cron.*and uncomment it.
This will cause all cron events to be logged to /var/log/cron.log (unless you changed the path) however the same events will also continue to be logged to syslog.
In the same file, find the line that looks like this;
Alter the line so that it looks like this;
Restart the logging service;
service rsyslog restart
Now cron will log to /var/log/cron.log but not to syslog