Docs: InternalsVisibleTo attribute docs incorrect

[dtchepak]

As per this SO comment, the How NSub Works docs are incorrect.

Use [assembly: InternalsVisibleTo(InternalsVisible.ToDynamicProxyGenAssembly2)] in the test assembly so that the internal member can be overridden.

should be:

Use [assembly: InternalsVisibleTo(InternalsVisible.ToDynamicProxyGenAssembly2)] in the assembly under test so that the internal member can be overridden.

[x789]

@dtchepak: Can you please assign this to me?

Apart from the proposed change, I would also explain in the documentation the possibility of using InternalsVisibleTo in csproj files (see dotnet/sdk#3439).