Update doc for parameter source_dir

[yangaws]

Issue #, if available:
#312

Description of changes:
README is not accurate for usage of source_dir. The entry point file should be in source_dir too.

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)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

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[codecov-io]

Codecov Report

Merging #323 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   92.92%   93.08%   +0.16%     
==========================================
  Files          51       51              
  Lines        3560     3560              
==========================================
+ Hits         3308     3314       +6     
+ Misses        252      246       -6

Impacted Files	Coverage Δ
src/sagemaker/local/image.py	89.44% <0%> (+1.75%)	⬆️

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