Fix the error that the file cannot be found when packaging the web

[arcticfox1919]

Thank you very much for your work.

But when I package the web version, I get a similar error:

Error: Error when reading '.../dartssh2-1.2.0-pre/lib/http_html.dart': The system cannot find the path specified.

I checked the code and found that it was because of the problem with the package path, so I fixed it.