Add missing arg to Tuner docstring

[laurenyu]

Description of changes:
One of the docstrings was missing an arg that got added later, though its usage is already documented in the README

Merge Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[codecov-io]

Codecov Report

Merging #319 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   92.92%   92.86%   -0.06%     
==========================================
  Files          50       50              
  Lines        3475     3475              
==========================================
- Hits         3229     3227       -2     
- Misses        246      248       +2

Impacted Files	Coverage Δ
src/sagemaker/tuner.py	96.8% <ø> (ø)	⬆️
src/sagemaker/local/image.py	88.85% <0%> (-0.59%)	⬇️

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