Update docs/configuring-playbook-synapse-simple-antispam.md: add the copyright header

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-synapse-simple-antispam.md

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+<!--
+SPDX-FileCopyrightText: 2019 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up Synapse Simple Antispam (optional, advanced)
 
 The playbook can install and configure [synapse-simple-antispam](https://github.com/t2bot/synapse-simple-antispam) for you.