Fix uses of default role in the docs

[AlexWaygood]

This is ReST, not MarkDown, so the default role is rendering as italics rather than as code in our docs: https://typing-extensions.readthedocs.io/en/latest/#sentinel-objects. Apparently sphinx-lint's default-role check, which would have flagged this, is disabled by default. I've tweaked our pre-commit configuration to enable that check, and filed sphinx-contrib/sphinx-lint#165 to suggest that sphinx-lint's documentation be improved to more clearly indicate which checks are enable/disabled by default.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (c461223) to head (928e123).

@@           Coverage Diff           @@
##             main     #779   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files           3        3           
  Lines        7860     7860           
=======================================
  Hits         7645     7645           
  Misses        215      215           

Flag	Coverage Δ
3.10	88.85% <ø> (ø)
3.10.4	88.85% <ø> (ø)
3.11	88.02% <ø> (ø)
3.11.0	87.27% <ø> (ø)
3.12	87.96% <ø> (ø)
3.12.0	87.95% <ø> (ø)
3.13	83.39% <ø> (ø)
3.13.0	84.09% <ø> (ø)
3.14	79.53% <ø> (ø)
3.9	89.53% <ø> (ø)
3.9.12	89.53% <ø> (ø)
pypy3.10	88.68% <ø> (ø)
pypy3.11	87.88% <ø> (ø)
pypy3.9	89.37% <ø> (ø)

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