Broken link to Ubuntu packages

[tredondo]

At https://getting-things-gnome.github.io/install, clicking "Ubuntu: Ubuntu GTG package²" leads to a "two or more packages specified (gtg lunar)" error page.

[nekohayo]

Basically the link should be modified to point to https://packages.ubuntu.com/search?keywords=gtg&searchon=names instead.

This needs to be done in the website's repository.