Fix typo in datamodel.rst (GH-6964)


This is a simple grammatical fix correcting "...object whose `__self__` attributes is ..." to "...object whose `__self__` attribute is ...".
(cherry picked from commit 00818c8ffd657f9ec727e366bfffd9c2135fa5ab)

Co-authored-by: Zach Mitchell <zmitchell@users.noreply.github.com>
1 file changed