Update LexResponse according to example and documentation.

[ezhdan-nyt]

Based on issue #197

Due to official documentation here: https://docs.aws.amazon.com/lex/latest/dg/lambda-input-response-format.html#using-lambda-response-format dialogAction field has to be required. Also, with implemented struct example in README_Lex.md will cause an error.

I have updated DialogAction parameter in LexResponse struct according to example and documentation.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #200 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #200   +/-   ##
======================================
  Coverage    77.2%   77.2%           
======================================
  Files          18      18           
  Lines         636     636           
======================================
  Hits          491     491           
  Misses        104     104           
  Partials       41      41

Impacted Files	Coverage Δ
events/lex.go	0% <ø> (ø)	⬆️

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