fix 1f offset

on the context,the offset of 1f should be 0x21.

Booting/linux-bootstrap-4.md

@@ -202,7 +202,7 @@ startup_32 (0x0)     +-----------------------+
                      +-----------------------+
 ```
 
-`startup_32` is linked to run at address `0x0` and this means that `1f` has the address `0x0 + offset to 1f`, approximately `0x22` bytes. The `ebp` register contains the real physical address of the `1f` label. So, if we subtract `1f` from the `ebp` we will get the real physical address of the `startup_32`. The Linux kernel [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) describes that the base of the protected mode kernel is `0x100000`. We can verify this with [gdb](https://en.wikipedia.org/wiki/GNU_Debugger). Let's start the debugger and put breakpoint to the `1f` address, which is `0x100022`. If this is correct we will see `0x100022` in the `ebp` register:
+`startup_32` is linked to run at address `0x0` and this means that `1f` has the address `0x0 + offset to 1f`, approximately `0x21` bytes. The `ebp` register contains the real physical address of the `1f` label. So, if we subtract `1f` from the `ebp` we will get the real physical address of the `startup_32`. The Linux kernel [boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) describes that the base of the protected mode kernel is `0x100000`. We can verify this with [gdb](https://en.wikipedia.org/wiki/GNU_Debugger). Let's start the debugger and put breakpoint to the `1f` address, which is `0x100021`. If this is correct we will see `0x100021` in the `ebp` register:
 
 ```
 $ gdb