Fix a italian translation

[fedeliallalinea]

Footer translation from
Built with Sphinx using a theme provided by Read the Docs.
to
Realizzato con Sphinx e il tema tema fornito da Read the Docs.
is wrong, the correct sentence would be
Realizzato con Sphinx usando un tema fornito da Read the Docs.

[benjaoming]

Hi @fedeliallalinea

Thanks for this! We do translations via Transifex: https://sphinx-rtd-theme.readthedocs.io/en/stable/contributing.html#translations

I've added your translation in Transifex. If you wish to be a part of the translation team, have a look here, you're more than welcome 🎉 https://www.transifex.com/readthedocs/sphinx-rtd-theme/