docs: kbuild: fix ReST formatting

Commit cd238eff ("docs: kbuild: convert docs to ReST and rename to
*.rst") missed a ReST header and a verbatim file content area.
Signed-off-by: Dov Murik <dovmurik@linux.vnet.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/modules.rst

@@ -182,7 +182,8 @@ module 8123.ko, which is built from the following files::
 	8123_pci.c
 	8123_bin.o_shipped	<= Binary blob
 
---- 3.1 Shared Makefile
+3.1 Shared Makefile
+-------------------
 
 	An external module always includes a wrapper makefile that
 	supports building the module using "make" with no arguments.