[mailman3] Fix the syntax error

I'm surprised this wasn't caught by yamllint.

Signed-off-by: Michal Konecny <mkonecny@redhat.com>

roles/mailman3/tasks/main.yml

@@ -89,7 +89,7 @@
 
 - name: Install needed packages
   ansible.builtin.package:
-    name: {{item}}
+    name: "{{item}}"
     state: present
   with_items:
     - httpd