Fix variable name typo

group_vars/matrix_servers

@@ -346,7 +346,7 @@ matrix_mx_puppet_skype_login_shared_secret: "{{ matrix_synapse_ext_password_prov
 # We don't enable bridges by default.
 matrix_mx_puppet_slack_enabled: false
 
-matrix_mx_puppet_skype_container_image_self_build: "{{ matrix_architecture != 'amd64'}}"
+matrix_mx_puppet_slack_container_image_self_build: "{{ matrix_architecture != 'amd64'}}"
 
 matrix_mx_puppet_slack_systemd_required_services_list: |
   {{