Resolved documentation issue with Code Verifier and Code Challenge

[petermcd]

Fixes #1322

Description of the Change

This resolves an issue with the instructions for generating a code challenge and code verifier. Following the instructions resulted in an error rather than getting the correct token details.

Checklist

• [x] PR only contains one change (considered splitting up PR)
• [x] unit-test added
• [x] documentation updated
• [x] CHANGELOG.md updated (only for user relevant changes)
• [x] author name in AUTHORS

[codecov[bot]]

Codecov Report

Merging #1323 (3cf78cd) into master (1eca949) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1323   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files          32       32           
  Lines        2073     2073           
=======================================
  Hits         2020     2020           
  Misses         53       53           

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