fix postinstall link

elabctl.sh

@@ -351,7 +351,7 @@ function install
         \Z1====>\Zn Then import the database structure with: \Zb\Z4elabctl initialize\Zn\n\n
         \Z1====>\Zn Go to https://$servername once started!\n\n
         In the mean time, check out what to do after an install:\n
-        \Z1====>\Zn https://doc.elabftw.net/postinstall.html\n\n
+        \Z1====>\Zn https://doc.elabftw.net/sysadmin-guide.html#setting-up-email\n\n
         The configuration file for docker-compose is here: \Z4$CONF_FILE\Zn\n
         Your data folder is: \Z4${DATA_DIR}\Zn. It contains the MySQL database and uploaded files.\n
         You can use 'docker logs -f ${ELAB_WEB_CONTAINER_NAME}' to follow the starting up of the container.\n" 20 80