docs: fix grammar in script messages

[Avicennasis]

Summary

Fix 3 grammar issues across GMP scripts:

• creates a alert → creates an alert (create-alerts-from-csv)
• exist, not creating → exists, not creating (create-filters-from-csv, create-targets-from-csv)

No functional changes — string messages only.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.27%. Comparing base (e2667d0) to head (16316df).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1370   +/-   ##
=======================================
  Coverage   49.27%   49.27%           
=======================================
  Files          18       18           
  Lines        1238     1238           
=======================================
  Hits          610      610           
  Misses        628      628           

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