kbuild: correct formatting of header in kbuild module docs

Minor formatting fixup.

Fixes: cd238eff ("docs: kbuild: convert docs to ReST and rename to *.rst")
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/modules.rst

@@ -498,7 +498,8 @@ build.
 	will be written containing all exported symbols that were not
 	defined in the kernel.
 
---- 6.3 Symbols From Another External Module
+6.3 Symbols From Another External Module
+----------------------------------------
 
 	Sometimes, an external module uses exported symbols from
 	another external module. kbuild needs to have full knowledge of