Fix small typing error in sample overload

[thomasaarholt]

What is this PR about?
This fixes a small error in the pm.sample overload introduced in #6709, following a discussion over at pyright's repo.

Checklist

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Bugfixes

• Fixes small typing error in pm.sample

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📚 Documentation preview 📚: https://pymc--6743.org.readthedocs.build/en/6743/

[codecov]

Codecov Report

Merging #6743 (8e6c64e) into main (fbc62d5) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6743   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
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  Files          95       95           
  Lines       16204    16204           
=======================================
  Hits        14907    14907           
  Misses       1297     1297           

Impacted Files	Coverage Δ
pymc/sampling/mcmc.py	89.81% <ø> (ø)