Fix dotenv variable casing in HTTP files example

[VedikaGupt]

Fixes #36559

Summary

Corrected the casing of the dotenv expression in the HTTP files documentation to match the behavior in Visual Studio.

Details

• Changed {{$dotEnv USERNAME}} to {{$dotenv USERNAME}}
• This resolves HTTP0012 error when evaluating environment variables

Testing

Not applicable (documentation-only change)

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Internal previews

📄 File	🔗 Preview link
aspnetcore/test/http-files.md	Use .http files in Visual Studio 2022

[guardrex]

Thanks for this, @VedikaGupt.

Note that there's a mention in the paragraph above this that looks like it will require an update, too.

I've pinged Tom to review. He takes care of this particular article.

[VedikaGupt]

Thanks for the review!! @guardrex, @tdykstra