Fix Signal role using incorrect database string variable

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/823

roles/matrix-bridge-mautrix-signal/defaults/main.yml

@@ -59,7 +59,7 @@ matrix_mautrix_signal_database_connection_string: 'postgres://{{ matrix_mautrix_
 
 matrix_mautrix_signal_appservice_database: "{{
  	{
- 		'postgres': matrix_mautrix_facebook_database_connection_string,
+ 		'postgres': matrix_mautrix_signal_database_connection_string,
  	}[matrix_mautrix_signal_database_engine]
  }}"