[3.12] gh-129892: Doc: Remove unnecessary role directive in graphlib.py (GH-129896) (#129905)

gh-129892: Doc: Remove unnecessary role directive in graphlib.py (GH-129896)

Change `:exec:ValueError` to `ValueError` in `TopologicalSorter.done()` docstring
(cherry picked from commit c53730171f18b90202aa0918b3c05412222bb1ec)

Co-authored-by: Nikola Savic <76233425+nikolasavic3@users.noreply.github.com>