support/scripts/size-stats: fix copy/paste error in ArgumentParser

A copy/paste error in the ArgumentParser() constructor call disclosed
the fact that the author of the script has shamefully based his work
on the existing graph-build-time script. This commit fixes this
mistake, therefore hiding in a better way how size-stats was
vampirized from graph-build-time.

Reported-by: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>