typo fix: there is no %{S:2} defined in booth.spec

In line 140 of booth.spec, there is "install -m 644 %{S:2}", but
when build booth on SLE12 or openSUSE Leap42.1, it reports %{SOURCE2}
is not defined. I guess the file should be contrib/geo-cluster.fwd.
This patch can fix the build error.