Make yamllint happy

Fixup for ba51997f7b

roles/matrix-nginx-proxy/tasks/nginx-proxy/setup_metrics_auth.yml

@@ -1,3 +1,5 @@
+---
+
 # When we're dealing with raw htpasswd content, we just store it in the file directly.
 - name: Ensure matrix-metrics-htpasswd is present when generated from raw content (protecting /metrics/* URIs)
   copy: