Fix join of base url for windows machines

[TheHappyLemon]

Found this issue, when installing laser2 model. Because my spm file was XML with errors, I decided to check the script itself. Figured out that the link to download is formed incorrectly, because on windows os.path.join will use "" insted of "/"

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[TheHappyLemon]

I signed the CLA

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[avidale]

Thanks @TheHappyLemon for your fix!
Would you be so kind to confirm that with it, downloading and using LASER models works on Windows?
If it is the case, your PR would also close the issue #255.

[TheHappyLemon]

@avidale Yes, I do confirm that it successfully downloads on Windows now!

[avidale]

I confirm that laser_encoders didn't work on Windows without this fix, and it is working now.
Thanks for spotting and fixing this problem!