[O] Update org-agenda-ndays in org-mode documentation

[Khalfani Wadlington]

The documentation for org-agenda-custom-commands at https://orgmode.org/worg/org-tutorials/org-custom-agenda-commands.html uses  


org-agenda-ndays

in the customization instead of

org-agenda-span

org-agenda-ndays  is deprecated in org-version 9.

Can somebody update it because it took me an embarrassingly long time to figure out what was wrong with my customization.