Fix typo in PPO recipe

[SLR722]

as title

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1607

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As of commit fc0be0e with merge base 6b55c1d ():
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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.28%. Comparing base (6b55c1d) to head (fc0be0e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1607      +/-   ##
==========================================
- Coverage   72.27%   70.28%   -1.99%     
==========================================
  Files         290      290              
  Lines       14552    14570      +18     
==========================================
- Hits        10517    10241     -276     
- Misses       4035     4329     +294     

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