fix: update NuGet registry URL to org-scoped format

The base URL https://nuget.pkg.github.com/ returns HTTP 405 errors when
Dependabot tries to resolve packages. The correct format is the org-scoped
URL with the index.json endpoint.

Fixes #43529

Author: loekensgard

content/code-security/how-tos/secure-at-scale/configure-organization-security/manage-usage-and-access/giving-org-access-private-registries.md

@@ -41,7 +41,7 @@ You need to be an **organization owner** to set up access to private registries
 1. In the expanded list of secrets and variables, select **Private registries** to display the "Private Registries" page.
 1. Select **New private registry** to add access details for a private registry.
 1. Use the **URL** and **Type** fields to define the location and type of the registry:
-   * **URL** is the location where you access the private registry. For example, to use the {% data variables.product.prodname_registry %} registry for NuGet: `https://nuget.pkg.github.com/`.
+   * **URL** is the location where you access the private registry. For example, to use the {% data variables.product.prodname_registry %} registry for NuGet: `https://nuget.pkg.github.com/OWNER/index.json`.
    * **Type** is the type of registry.
 1. Select either **Token** or **Username and password**, depending on the authentication method, then enter data into the appropriate fields.
    Some types of authentication tokens, such as a {% data variables.product.github %} {% data variables.product.pat_generic_title_case %}, are tied to a particular user identity. Select the **Username and password** option for these and enter the relevant username as **Username** and the token as **Password**.