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>

Documentation/kbuild/makefiles.rst

@@ -1269,12 +1269,12 @@ When kbuild executes, the following steps are followed (roughly):
 	Example::
 
 		#arch/x86/kernel/Makefile
-		always := vmlinux.lds
+		extra-y := vmlinux.lds
 
 		#Makefile
 		export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
 
-	The assignment to $(always) is used to tell kbuild to build the
+	The assignment to extra-y is used to tell kbuild to build the
 	target vmlinux.lds.
 	The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
 	specified options when building the target vmlinux.lds.