use SIZEOF(OAM) instead of 160 in part2 oam clearing loop

[magnusKue]

in https://gbdev.io/gb-asm-tutorial/part2/objects.html the oam is cleared by storing 160 in b as a counter.
I'd suggest using SIZEOF(OAM) here to show its existence and because ENDOF(OAM) is used just one line below and its more verbose.

Just an idea, thank you for your great work!! :)