Switch Definition to Model

[dalumiller]

Issue #, if available:
The term "AWS Serverless Application Definition template" is not one I have seen anywhere else in this doc nor on any other doc, but "AWS Serverless Application Model template" is found. I'm assuming this is a prior naming convention that should be updated to prevent confusion.

Description of changes:
Changing two instances of the word "Definition" to "Model"

Description of how you validated changes:
n/a

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #1423 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1423   +/-   ##
======================================
  Coverage    94.4%   94.4%           
======================================
  Files          78      78           
  Lines        4556    4556           
  Branches      911     911           
======================================
  Hits         4301    4301           
  Misses        120     120           
  Partials      135     135

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