Typo fix in air-gapped.asciidoc

[rseldner]

Small PR to replace windows_x86_64 with windows-x86_64

$ curl -I https://artifacts.elastic.co/downloads/beats/elastic-agent/elastic-agent-8.12.1-windows_x86_64.zip          
-HTTP/2 404 
date: Tue, 11 Feb 2025 21:29:27 GMT
strict-transport-security: max-age=31536000; includeSubDomains
via: 1.1 google
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000

$ curl -I https://artifacts.elastic.co/downloads/beats/elastic-agent/elastic-agent-8.12.1-windows-x86_64.zip
+HTTP/2 200 
date: Tue, 11 Feb 2025 21:30:57 GMT
content-type: application/zip
content-length: 294014409
accept-ranges: bytes
last-modified: Tue, 06 Feb 2024 09:25:28 GMT
etag: "078e7b61ea7aecc867bd8ceb81a986e2"
cache-control: public, max-age=600
strict-transport-security: max-age=31536000; includeSubDomains
via: 1.1 google
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[mergify]

This pull request does not have a backport label. Could you fix it @rseldner? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[rseldner]

will resubmit to version branches per https://elastic.github.io/docs-builder/migration/freeze/index.html