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>

Documentation/kbuild/makefiles.rst

@@ -1509,7 +1509,7 @@ When kbuild executes, the following steps are followed (roughly):
 	the linked vmlinux file.
 
 8 Kbuild syntax for exported headers
-------------------------------------
+====================================
 
 The kernel includes a set of headers that is exported to userspace.
 Many headers can be exported as-is but other headers require a