Do you really think the person you are replying to doesn't know that already? I think you can take it as given that most people here know the difference between MHz and mHz and are perfectly capable of realising it is a typo.
I wouldn't be so sure. You can go pretty far without knowing what a MHz really means.
Edit: I've it megahertz written more as mhz than MHZ.
If someone doesn't know what it stands for, that's also not a problem. You would have to have an extremely specific partial understanding to both notice the capitalization difference and not realize it's the same thing.