update macports repo path

INSTALL

@@ -94,10 +94,10 @@ In order to install Kwant, you have to
 
 2. After the installation, open a terminal and execute ::
 
-       echo http://dl.dropboxusercontent.com/u/769003/ports.tar | sudo tee -a /opt/local/etc/macports/sources.conf >/dev/null
+       echo http://downloads.kwant-project.org/macports/ports.tar | sudo tee -a /opt/local/etc/macports/sources.conf >/dev/null
 
    (this adds the Kwant MacPorts download link
-   `<http://dl.dropboxusercontent.com/u/769003/ports.tar>`_ at the end of the
+   `<http://downloads.kwant-project.org/macports/ports.tar>`_ at the end of the
    ``sources.conf`` file.)
 
 3. Execute ::