docs/kbuild/makefiles: fix header underline

The "Kbuild syntax for exported headers" section should not be under
"Architecture Makefiles" in hierarchy. Bump the header underline from
"-" to "=".

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>