Fix the name of the placeholder file

[ericstj]

This doc should not reccomend that users create a . file. That's a reserved directory alias for current directory. I'd be surprised if that actually works, it would definitely confuse the build / extraction / etc. I can't even create a file with this name. The correct file name is _._.

cc @JonDouglas @zivkan

[learn-build-service-prod]

Learn Build status updates of commit cb47372:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/reference/nuspec.md	✅Succeeded	View

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