merge in right archlinux package, thanks to @saram-kon from...

merge in right archlinux package, thanks to @saram-kon from https://github.com/matrix-org/synapse/pull/1956

contrib/systemd/synapse.service

 # This assumes that Synapse has been installed as a system package
-# (e.g. https://aur.archlinux.org/packages/matrix-synapse/ for ArchLinux)
+# (e.g. https://www.archlinux.org/packages/community/any/matrix-synapse/ for ArchLinux)
 # rather than in a user home directory or similar under virtualenv.
 
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