Include .NET Framework 4.8.1 in .NET Standard documentation

[joeltankam]

I think the doc should mention that .NET Standard 2.0 is (also) compatible with .NET Framework 4.8.1.

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Document Details

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• ID: 9eb8b774-c6f0-8235-6964-15219baf2ea7
• Version Independent ID: 6819a4a7-7527-d8c7-e132-767f7344e00e
• Content: .NET Standard
• Content Source: docs/standard/net-standard.md
• Product: dotnet
• Technology: dotnet-standard
• GitHub Login: @gewarren
• Microsoft Alias: gewarren