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 <[email protected]>

Author: zdover23

doc/cephadm/services/osd.rst

@@ -110,7 +110,7 @@ general form of the command above:
 
    "3.64 TB"
 
-This means that the exact device size is 3.64 * 1000, or 3640GB.
+This means that the exact device size is 3.64 TB, or 3640GB.
 
 This procedure was developed by Frédéric Nass. See `this thread on the
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