Fix username_template potentially not being taken into account for mautrix-slack

While working on upgrading the Meta bridges to bridgev2, I've noticed
that {% raw %} and {% endraw %} on lines like that (immediately
preceding `username_template` may cause YAML indentation issues.

roles/custom/matrix-bridge-mautrix-slack/templates/config.yaml.j2

@@ -244,9 +244,7 @@ appservice:
     hs_token: {{ matrix_mautrix_slack_homeserver_token | to_json }}
 
     # Localpart template of MXIDs for remote users.
-    # {% raw %}
-    # {{.}} is replaced with the internal ID of the user.
-    # {% endraw %}
+    # {% raw %}{{.}}{% endraw %} is replaced with the internal ID of the user.
     username_template: "{% raw %}slack_{{.}}{% endraw %}"
 
 # Config options that affect the Matrix connector of the bridge.