Fix off-by-one NUTS adaptations

[penelopeysm]

Closes #2750.

Prior to #2674 the counter i would be initialised to 1 after the first adaptation had happened. That PR, first released in v0.41, changed it such that the counter i was initialised to 1 before the first adaptation happened. That should, of course, be zero, which this PR fixes.

I've checked locally and this change makes it such that results from pre-v0.41 are exactly reproducible now, as long as all else is equal (e.g. initialisation, discard_initial, etc).

[github-actions]

Turing.jl documentation for PR #2751 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2751/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.65%. Comparing base (2b95583) to head (345db50).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2751   +/-   ##
=======================================
  Coverage   86.65%   86.65%           
=======================================
  Files          20       20           
  Lines        1259     1259           
=======================================
  Hits         1091     1091           
  Misses        168      168           

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