Rename nightly build GitHub source in FAQS.md to be consistent

[jeffhandley]

Motivation and Context

While configuring my machine to get nightly builds, I noticed an inconsistency in the FAQ for configuring the GitHub package source.

Description

Updates the NuGet.config illustrated to use the same package source name referenced in the powershell statement.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull Request Overview

This PR updates the NuGet configuration example in the FAQ documentation to use a more descriptive package source key name. The change improves consistency with the command-line example shown earlier in the same section.

• Renames the package source key from "github" to "GitHubMicrosoft" throughout the NuGet.Config XML example
• Improves XML indentation consistency in the packageSourceCredentials section
• Ensures the manual configuration example matches the dotnet nuget add source command which already uses the GitHubMicrosoft name