Fix the typo

'0xffff' --> '0xfffff'

Booting/linux-bootstrap-3.md

@@ -454,7 +454,7 @@ The `GDT_ENTRY_BOOT_CS` has index - 2 here, `GDT_ENTRY_BOOT_DS` is `GDT_ENTRY_BO
 * limit - 0xfffff
 * flags - 0xc09b
 
-What does this mean? The segment's base address is 0, and the limit (size of segment) is - `0xffff` (1 MB). Let's look at the flags. It is `0xc09b` and it will be:
+What does this mean? The segment's base address is 0, and the limit (size of segment) is - `0xfffff` (1 MB). Let's look at the flags. It is `0xc09b` and it will be:
 
 ```
 1100 0000 1001 1011