Fix matrix_synapse_encryption_enabled_by_default_for_room_type

The value of `off` was taken to be a boolean, but it shouldn't be.

Synapse expects a string (currently one of: `all`, `invite`, `off`).

roles/matrix-synapse/defaults/main.yml

@@ -575,7 +575,7 @@ matrix_synapse_spam_checker: []
 
 matrix_synapse_modules: []
 
-matrix_synapse_encryption_enabled_by_default_for_room_type: false
+matrix_synapse_encryption_enabled_by_default_for_room_type: "off"
 
 matrix_synapse_trusted_key_servers:
   - server_name: "matrix.org"