Documentation: kbuild: fix the section title format

Make it consistent with the other sections.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -1241,7 +1241,8 @@ When kbuild executes, the following steps are followed (roughly):
 	will be displayed with "make KBUILD_VERBOSE=0".
 
 
---- 6.9 Preprocessing linker scripts
+6.9 Preprocessing linker scripts
+--------------------------------
 
 	When the vmlinux image is built, the linker script
 	arch/$(ARCH)/kernel/vmlinux.lds is used.