Fix the dash typo in lang flag of classlib in Unit testing Visual Basic page

[BartoszKlonowski]

This pull request fixes #46461 by correcting mistakes reported in the issue:

• removes a dash typo from the -lang option
• adds the -lang option to two dotnet new xunit calls
  By default, this command creates the C# project, but this page is about the VB, hence adding this -lang option to make sure VB project is created seems logical.

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-visual-basic-with-xunit.md	Unit testing Visual Basic .NET Core libraries using dotnet test and xUnit

[BillWagner]

Thanks @BartoszKlonowski

This LGTM, and I'll now.