init/Kconfig: fix LOCALVERSION_AUTO help text

It was never guaranteed to be exactly eight, but since commit
548b8b51 ("scripts/setlocalversion: make git describe output more
reliable"), it has been exactly 12.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>