fastboot: Remove "bootloader-version" variable

As per [1], there is no such fastboot variable as "bootloader-version".
Only "version-bootloader" is supported. Let's reflect this and not
confuse users further.

[1] https://android.googlesource.com/platform/system/core/+/refs/tags/android-q-preview-4/fastboot/README.md

Fixes: 3aab70afc531d1 ("usb/gadget: add the fastboot gadget")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
diff --git a/doc/android/fastboot.txt b/doc/android/fastboot.txt
index ea0d1da..9de1322 100644
--- a/doc/android/fastboot.txt
+++ b/doc/android/fastboot.txt
@@ -169,9 +169,9 @@
 
 ::
 
-   $ fastboot getvar bootloader-version
-   bootloader-version: U-Boot 2014.04-00005-gd24cabc
-   finished. total time: 0.000s
+   $ fastboot getvar version-bootloader
+   version-bootloader: U-Boot 2019.07-rc4-00240-g00c9f2a2ec
+   Finished. Total time: 0.005s
 
 or initiate a reboot: