Add missing trailing period.

[aslamdoctor]

What?

Closes
#127

Add trailing period to error message.

Why?

How?

Testing Instructions

1. install plugin
2. activate plugin
3. go to Settings > AI Experiments and see error message

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: aslamdoctor <aslamdoctor@git.wordpress.org>
Co-authored-by: mindctrl <mindctrl@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.33%. Comparing base (8cc94c8) to head (07e92e1).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/Settings/Settings_Page.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #132   +/-   ##
==========================================
  Coverage      27.33%   27.33%           
  Complexity       152      152           
==========================================
  Files             14       14           
  Lines            900      900           
==========================================
  Hits             246      246           
  Misses           654      654           

Flag	Coverage Δ
unit	27.33% <0.00%> (ø)

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[mindctrl]

I noticed this yesterday. Thanks for the quick fix.