Fix typos in docs/extend developer documentation

[Copilot]

Fix four spelling/grammar errors in developer documentation under docs/extend/:

• vizualizations → visualizations in export-dashboards.md
• perfomed running → performed by running in testing.md
• writen → written in filebeat-modules-devguide.md
• custome → custom in event-fields-yml.md

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @Copilot? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[strawgate]

Simple spelling fixes, looks good.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

🔍 Preview links for changed docs

• docs/extend/event-fields-yml.md
• docs/extend/export-dashboards.md
• docs/extend/filebeat-modules-devguide.md
• docs/extend/testing.md

[github-actions]

Vale Linting Results

Summary: 4 warnings, 2 suggestions found

⚠️ Warnings (4)

File	Line	Rule	Message
docs/extend/event-fields-yml.md	166	Elastic.BritishSpellings	Use American English spelling 'behavior' instead of British English 'behaviour'.
docs/extend/export-dashboards.md	42	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'and so on' instead of 'etc'.
docs/extend/filebeat-modules-devguide.md	433	Elastic.DontUse	Don't use 'just'.
docs/extend/testing.md	64	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'that is' instead of 'i.e'.

💡 Suggestions (2)

File	Line	Rule	Message
docs/extend/filebeat-modules-devguide.md	431	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
docs/extend/testing.md	64	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.

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