Update README to reflect user-facing name
Currently the README refers to the application, in terms of its name, as "Nautilus", not the current name of "Files". AIUI "nautilus" is just the project name now.
Also, the link in the summary points to https://wiki.gnome.org/Apps/Nautilus, which no longer exists - current page is https://wiki.gnome.org/action/show/Apps/Files.