docs: fix Hessian docstrings

[adrhill]

Applies two fixes to the Hessian and HVP docs:

• Docstrings for prepare!_hessian and prepare!_hvp were missing the prep argument
• prepare!_hvp called docstring_prepare instead of docstring_prepare!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.45%. Comparing base (505df72) to head (fc8389d).

❗ There is a different number of reports uploaded between BASE (505df72) and HEAD (fc8389d). Click for more details.

HEAD has 24 uploads less than BASE

Flag	BASE (505df72)	HEAD (fc8389d)
DI	51	28
DIT	12	11

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #968       +/-   ##
===========================================
- Coverage   98.21%   87.45%   -10.76%     
===========================================
  Files         135      134        -1     
  Lines        7995     7963       -32     
===========================================
- Hits         7852     6964      -888     
- Misses        143      999      +856     

Flag	Coverage Δ
DI	84.04% <ø> (-14.94%)	⬇️
DIT	96.22% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (505df72) to head (fc8389d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #968   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files         135      135           
  Lines        7995     7995           
=======================================
  Hits         7852     7852           
  Misses        143      143           

Flag	Coverage Δ
DI	98.97% <ø> (ø)
DIT	96.22% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.