Fix max_completion_tokens

[alexsacr]

The json tag is incorrect, and results in an error from the API when using the o1 model.

I didn't modify the struct field name to maintain compatibility if anyone else had started using it, but it wouldn't work for them either.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.03%. Comparing base (774fc9d) to head (c15f8bb).
Report is 53 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #860      +/-   ##
==========================================
+ Coverage   98.46%   99.03%   +0.57%     
==========================================
  Files          24       26       +2     
  Lines        1364     1454      +90     
==========================================
+ Hits         1343     1440      +97     
+ Misses         15        8       -7     
  Partials        6        6              

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[l-winston]

lgtm

[sashabaranov]

Thank you!