Update the Nebulab's logo on README.md

After Nebulab's website update, we don't have anymore the path with public logos (and we've updated the logo, too). This PR updates the URL of the logo image and the URL of the website (`.com`), too.

README.md

@@ -42,7 +42,7 @@ As a community-driven project, Solidus relies on funds and time donated by devel
 ### Main Contributor & Director
 At present, Nebulab is the main code contributor and director of Solidus, providing technical guidance and coordinating community efforts and activities.
 
-[![Nebulab](https://nebulab.it/assets/images/public/logo.svg)](https://nebulab.it/)
+[![Nebulab](https://nebulab.com/assets/img/logo-nebulab_black.svg)](https://nebulab.com/)
 
 ### Ambassadors
 Support this project by becoming a Solidus Ambassador. Your logo will show up here with a link to your website. [Become an Ambassador](https://opencollective.com/solidus).