fix: Fixed broken image in README.md

* Fixed the image of lubuntu in README.md. The bug was introduced in
  https://github.com/notsecure/uTox/commit/0ae8b94183b6ec2321dadd97b2c9da64e14f7013
  It seems that the file has been renamed but was not `git add`-ed. This
  commit fixes the problem.