Change 'distribution' to 'distributions' in documentation

[laurenyu]

Description of changes:
It was actually supposed to be "distribution" but even though the code is "wrong" and the docs are "right", we should stick with the "wrong" name for backward compatibility 😭

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[codecov-io]

Codecov Report

Merging #503 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
+ Coverage    92.7%   92.82%   +0.11%     
==========================================
  Files          71       71              
  Lines        5336     5336              
==========================================
+ Hits         4947     4953       +6     
+ Misses        389      383       -6

Impacted Files	Coverage Δ
src/sagemaker/mxnet/estimator.py	100% <ø> (ø)	⬆️
src/sagemaker/tensorflow/estimator.py	94.65% <ø> (ø)	⬆️
src/sagemaker/local/image.py	89.6% <0%> (+1.83%)	⬆️

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[jesterhazy]

you could add a new parameter 'distribution' and change the code to work with either. and add a deprecation warning on 'distributions'

[jesterhazy]

let's do a new PR later to add 'distribution' and change these docs back.