fix description of a20_test()

Booting/linux-bootstrap-3.md

@@ -335,7 +335,9 @@ static int a20_test(int loops)
 
 First of all, we put `0x0000` in the `FS` register and `0xffff` in the `GS` register. Next, we read the value at the address `A20_TEST_ADDR` (it is `0x200`) and put this value into the variables `saved` and `ctr`.
 
-Next, we write an updated `ctr` value into `fs:gs` with the `wrfs32` function, then delay for 1ms, and then read the value from the `GS` register into the address `A20_TEST_ADDR+0x10`, if it's not zero we've already enabled the A20 line. If A20 is disabled, we try to enable it with a different method which you can find in `a20.c`. For example, it can be done with a call to the `0x15` BIOS interrupt with `AH=0x2041`.
+Next, we write an updated `ctr` value into `fs:A20_TEST_ADDR` or `fs:0x200` with the `wrfs32` function, then delay for 1ms, and then read the value from the `GS` register into the address `A20_TEST_ADDR+0x10`. In a case when `a20` line is disabled, the address will be overlapped, in other case if it's not zero `a20` line is already enabled the A20 line.
+
+If A20 is disabled, we try to enable it with a different method which you can find in `a20.c`. For example, it can be done with a call to the `0x15` BIOS interrupt with `AH=0x2041`.
 
 If the `enabled_a20` function finished with a failure, print an error message and call the function `die`. You can remember it from the first source code file where we started - [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/boot/header.S):