Context: Canadian tech company
If the email is to me, and me alone, I would respond with:
Ok, thanks for telling me.
or, if I think they will need me to do something while they are sick:
Ok, do you need me to cover anything while you are out?
If the email is to a group, like the department or company, I wouldn't respond at all. The email has done it'...
A very simple message will do. Don't "make it weird" like Hristo Kolev said. Something like...
I am sorry to hear about your sickness. Let me know if there's
anything I can do for you.
Feel better soon!
Don't overthink it. "Feel better soon!" is fine. You can also clarify if there's anything special your boss needs you to handle while they are recovering. This is what I usually reply to emails like that,
Sorry to hear that, I hope you feel better soon. Let me know if there's anything I can handle for you here in the office while you're out.