kbuild: fix the document to use extra-y for vmlinux.lds

kbuild: fix the document to use extra-y for vmlinux.lds

The difference between "always" and "extra-y" is that the targets
listed in $(always) are always built, whereas the ones in $(extra-y)
are built only when KBUILD_BUILTIN is set.

So, "make modules" does not build the targets in $(extra-y).

vmlinux.lds is only needed for linking vmlinux. So, adding it to extra-y
is more correct. In fact, arch/x86/kernel/Makefile does this.

Fix the example code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>