service: rename to "Local Caching Server"

The Description functions as a title.
systemd.unit(5) says:
> systemd may use this string as a noun in status messages
> ("Starting description...", "Started description.",
> "Reached target description.", "Failed to start description."),
> so it should be capitalized, and should not be a full sentence,
> or a phrase with a continuous verb.

data/passim.service.in

@@ -1,5 +1,5 @@
 [Unit]
-Description=A local caching server
+Description=Local Caching Server
 Documentation=https://github.com/hughsie/passim
 After=avahi-daemon.service
 Before=display-manager.service