etc: Fix misleading comment in discosrv options file (#6258)

cmd/stdiscosrv/etc/linux-systemd/default

@@ -1,3 +1,3 @@
-# Default settings for syncthing-relaysrv (strelaysrv).
+# Default settings for syncthing-discosrv (stdiscosrv).
 ## Add Options here:
 DISCOSRV_OPTS=