Fix typo.

03-bootsector-memory/boot_sect_memory.asm

@@ -43,7 +43,7 @@ jmp $ ; infinite loop
 
 the_secret:
     ; ASCII code 0x58 ('X') is stored just before the zero-padding.
-    ; On this code that is at byte 0x2d (check it out using 'xdd file.bin')
+    ; On this code that is at byte 0x2d (check it out using 'xxd file.bin')
     db "X"
 
 ; zero padding and magic bios number

03-bootsector-memory/boot_sect_memory_org.asm

@@ -39,7 +39,7 @@ jmp $ ; infinite loop
 
 the_secret:
     ; ASCII code 0x58 ('X') is stored just before the zero-padding.
-    ; On this code that is at byte 0x2d (check it out using 'xdd file.bin')
+    ; On this code that is at byte 0x2d (check it out using 'xxd file.bin')
     db "X"
 
 ; zero padding and magic bios number