Fix installation instructions (matches Ubuntu 18.04 Tweak tool)
Created by: benjaoming
Renamed tarball to .tar.gz
-- seems okay?
Changed instructions because
- You cannot install an extension with the Tweak tool anymore. At least not on Ubuntu 18.04
- The tarball does not contain a root directory so unpacking directly in
~/.local/share/gnome-shell/extensions/
will put all build outputs there, and not in a correct subdir.