Add bot:prefix: to template

roles/custom/matrix-bot-matrix-registration-bot/templates/config.yaml.j2

@@ -10,6 +10,7 @@ bot:
   server: {{ matrix_bot_matrix_registration_bot_bot_server|to_json }}
   username: {{ matrix_bot_matrix_registration_bot_matrix_user_id_localpart|to_json }}
   password: {{ matrix_bot_matrix_registration_bot_bot_password|to_json }}
+  prefix: {{ matrix_bot_matrix_registration_bot_bot_prefix|to_json }}
 
 api:
   # API endpoint of the registration tokens