kconfig2html: Denote that the script is python3

`python` as a command isn't universally available anymore after the
python2/python3 drama.

Change-Id: I9d68873d86dc3f044238d921c10fc434a83a76f5
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69190
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

util/docker/coreboot.org-status/kconfig2html

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # kconfig2wiki - Kconfig to MediaWiki converter for
 # https://www.coreboot.org/coreboot_Options