fix README for PyPI

[ChoiByungWook]

Issue #, if available:
The README doc is broken in PyPI: https://pypi.org/project/sagemaker/1.18.3/

The code snippet below was missing a newline.

Twine provides the ability to check if your readme will render properly on PyPI.
https://packaging.python.org/guides/making-a-pypi-friendly-readme/#validating-restructuredtext-markup

aws/sagemaker-tensorflow-extensions#32

Description of changes:
Added newline in code snippet portion in README.
Bumped to 1.18.3.post1

Going to add the step in our CI.

=== TEST ===
twine check dist/*
/anaconda3/lib/python3.6/site-packages/readme_renderer/markdown.py:38: UserWarning: Markdown renderers are not available. Install 'readme_render[md]' to enable Markdown rendering.
warnings.warn(_EXTRA_WARNING)
Checking distribution dist/sagemaker-1.18.3.post1.tar.gz: Passed

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)
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[codecov-io]

Codecov Report

Merging #643 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
+ Coverage   92.75%   92.77%   +0.01%     
==========================================
  Files          71       71              
  Lines        5465     5465              
==========================================
+ Hits         5069     5070       +1     
+ Misses        396      395       -1

Impacted Files	Coverage Δ
src/sagemaker/amazon/common.py	97.01% <0%> (+0.74%)	⬆️

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