minor formatting for yaml in certbot role

roles/certbot/defaults/main.yaml

@@ -1,2 +1,2 @@
 ---
-  webserver_name: nginx
+webserver_name: nginx

roles/certbot/tasks/main.yaml

@@ -1,8 +1,9 @@
+---
 - name: install certbot
   apt:
     name: "{{ item }}"
     state: present
-    update_cache: yes
+    update_cache: true
     cache_valid_time: 3600
   with_items:
     - certbot