chore(contrib): update for Community -> Extra merge on Arch Linux (#938)

The Arch Linux `syncthing` package is now in the Extra repository
instead of Community, because they have been
[merged](https://rfc.archlinux.page/0014-merge-package-repos).

Author: marbens-arch

users/contrib.rst

@@ -136,7 +136,7 @@ Unofficial `RPM repo of Syncthing <https://copr.fedorainfracloud.org/coprs/dafta
 ArchLinux
 ~~~~~~~~~
 
-- Official Community Repository: `syncthing <https://archlinux.org/packages/?name=syncthing>`__
+- Official Extra Repository: `syncthing <https://archlinux.org/packages/?name=syncthing>`__
 
 - Arch User Repository: `syncthingtray <https://aur.archlinux.org/packages/syncthingtray>`__