manual: Add publishing-function to publishing example

* doc/org-manual.org (Example: simple publishing configuration): Add
:publishing-function to org-publish-project-alist.

This appears to have been necessary since 0ccf650b4 (org-e-publish:
Remove default value for publishing function, 2012-10-08).

Reported-by: dalanicolai <dalanicolai@gmail.com>
Ref: https://orgmode.org/list/CACJP=3k2jL1z0d-uALFGoutffFF2o=7-L39zUns6si0KAhZ-_g@mail.gmail.com

doc/org-manual.org

@@ -16248,6 +16248,7 @@ directory on the local machine.
 (setq org-publish-project-alist
       '(("org"
          :base-directory "~/org/"
+         :publishing-function org-html-publish-to-html
          :publishing-directory "~/public_html"
          :section-numbers nil
          :table-of-contents nil