Update broken Polly example

[SamRemis]

Fixes Boto3 #4920

The examples in Polly's docs is misleading; the current suggested input is for content is file://example.pls, but that will cause the literal string of file://example.pls to be uploaded as the content, resulting in the following error:

InvalidLexiconException: An error occurred (InvalidLexiconException) when calling the PutLexicon operation: Invalid PLS Lexicon

Polly expects this input to be a lexicon as defined by the PLS specificiation. This PR is to make it clearer that we will not open and retrieve the file contents on behalf of the customer, and that they must provide the contents of the pls directly.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.18%. Comparing base (b1279af) to head (446ae42).
Report is 99 commits behind head on develop.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3300   +/-   ##
========================================
  Coverage    93.18%   93.18%           
========================================
  Files           66       66           
  Lines        14345    14375   +30     
========================================
+ Hits         13367    13396   +29     
- Misses         978      979    +1     

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[alexgromero]

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