doc: avtools-common-opts: Fix terminology concerning metric prefixes

'k', 'M', and 'G' are SI (unit) prefixes or metric prefixes,
not 'number postfixes'. Also, the statement regarding binary
prefixes ("powers of 2 are used instead of powers of 10")
might be misinterpreted (1 kB = 10^3 B, but 1 KiB != 2^3 B).

Signed-off-by: Diego Biurrun <diego@biurrun.de>