etc: Update systemd docs URL

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GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4145

etc/linux-systemd/README.md

@@ -5,4 +5,4 @@ This directory contains configuration files for running Syncthing under the
 systemd user service. For further documentation take a look at the [systemd
 section][1] on https://docs.syncthing.net.
 
-[1]: https://docs.syncthing.net/users/autostart.html#systemd
+[1]: https://docs.syncthing.net/users/autostart.html#using-systemd