Aspnetcore.docs: Add a colon separator may not work on all platforms

[Tiberriver256]

The instructions state:

Set the Authentication:Microsoft:ClientIdand Authentication:Microsoft:ClientSecret as application settings in the Azure portal. The configuration system is set up to read keys from environment variables.

Attempting to set these as application settings in the Azure Portal for a Linux App Service results in the following message:

App setting names can only contain letters, numbers (0-9), periods ("."), and underscores ("_")

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Document Details

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• ID: a4c264c0-0c5c-298e-bb45-9b4891c8d162
• Version Independent ID: ce69b990-0b4c-abda-cd2d-68f85cd8031e
• Content: Microsoft Account external login setup with ASP.NET Core
• Content Source: aspnetcore/security/authentication/social/microsoft-logins.md
• Product: aspnet-core
• Technology: aspnetcore-security
• GitHub Login: @Rick-Anderson
• Microsoft Alias: riande

[Rick-Anderson]

In environment variables, a colon separator may not work on all platforms. A double underscore, __, is supported by all platforms and is automatically converted into a colon. See [Keys](xref:fundamentals/configuration#keys) for more info.

Need an include with the preceding in all the docs in this series.

[Rick-Anderson]

Those instructions are in the article.