manual: minor fixes in the graph-size documentation

Fix some typos and references to a size-stats 'target' (the script is called
'size-stats' but the make target is 'graph-size').

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>