[pg.211][Code] DISK_IMAGE path error

[7LH0rse]

In page 211, Makefile, there's a line of code:

DISK_IMG=disk.img

According to Figure 7.7.4, disk.img should be in build/.
Using this wrong path would cause disk.img generated in ./, but not in build/.
Correction:

DISK_IMG=$(BUILD_DIR)/disk.img