doc/cephadm: simplify confusing math proposition

doc/cephadm: simplify confusing math proposition

s/This means that the exact device size is 3.64 * 1000, or 3640GB"/This
means that the exact device size is 3.64TB, or 3640 GB"/

In the original text, the number "3.64" appears to refer to a quantity
(and indeed, it is a quantity of Terabytes), but it is unlabeled. Also,
on repeated recent readings of this sentence I found it more puzzling
than enlightening. So I made this commit.

Signed-off-by: Zac Dover <zac.dover@proton.me>