Fix type error

[MasashiYokota]

I've fixed a type error on Tesla.MultiPart.add_file/3.

I've simply removed its option because there is no :read option for File.stream!/3.
If this patch has any problems. please let me know.

Thank you!

[tanguilp]

Thank you @MasashiYokota

This incorrect parameter causes Dialyzer to emit warnings in projects using Tesla. Surprisingly, it doesn;t when running it on the tesla library alone.

:read isn't a valid option indeed for streaming files, as specified here: https://hexdocs.pm/elixir/1.11.3/File.html#stream!/3

[teamon]

Thanks!