fix offsets

bootmain.c

 // Boot loader.
 // 
-// The BIOS loads boot sector (bootasm.S) from sector 1 of the disk
+// The BIOS loads boot sector (bootasm.S) from sector 0 of the disk
 // into memory and executes it.  The boot sector puts the processor
 // in 32-bit mode and calls bootmain below, which loads an ELF kernel
-// image from the disk starting at sector 2 and then jumps to the
+// image from the disk starting at sector 1 and then jumps to the
 // kernel entry routine.
 
 #include "types.h"