fix: update NuGet registry URL to org-scoped format

[loekensgard]

Why:

Closes: #43529

The documentation example uses https://nuget.pkg.github.com/ as the NuGet feed URL, but this base URL causes HTTP 405 errors when Dependabot tries to resolve packages, resulting in silently skipped updates. The org-scoped format https://nuget.pkg.github.com/OWNER/index.json is the correct URL.

What's being changed:

Updated the example NuGet feed URL in the "Defining registry access for Code Scanning default setup" section from https://nuget.pkg.github.com/ to https://nuget.pkg.github.com/OWNER/index.json.
Used OWNER to match the placeholder convention elsewhere in the NuGet registry docs, but happy to change to ORGANIZATION if that reads more clearly here.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
code-security/how-tos/secure-at-scale/configure-organization-security/manage-usage-and-access/giving-org-access-private-registries.md	fpt ghec ghes@ 3.20 3.19 3.18	fpt ghec ghes@ 3.20 3.19 3.18

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

Updates the NuGet feed URL example in the private registries configuration docs to use the org-/owner-scoped NuGet v3 index endpoint, aligning the documentation with the URL format Dependabot can successfully resolve.

Changes:

• Replaces the NuGet registry example URL from the base host to an owner-scoped /OWNER/index.json URL.

[Sharra-writes]

@loekensgard Looks good! Thanks for the fix.

[github-actions]

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