Fix undefined variable reference

roles/matrix-bridge-mx-puppet-discord/templates/config.yaml.j2

@@ -100,7 +100,7 @@ namePatterns:
   group: :name
 
 database:
-{% if matrix_mx_puppet_slack_discord_database_engine == 'sqlite' %}
+{% if matrix_mx_puppet_discord_database_engine == 'sqlite' %}
   # Use SQLite3 as a database backend
   # The name of the database file
   filename: /data/database.db