fix minor typos in authentication_guide.md

[lusentis]

• added a missing "-" in a domain name
• replaced "them" with "theme"

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

@@           Coverage Diff           @@
##           master     #688   +/-   ##
=======================================
  Coverage   87.17%   87.17%           
=======================================
  Files          72       72           
  Lines        3424     3424           
  Branches      648      648           
=======================================
  Hits         2985     2985           
  Misses        422      422           
  Partials       17       17

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[mbahar]

Good catches! Thanks for the contribution!

[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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