mailman3: Add missing trailing slash for the /static/ path

The code isn't vulnerable to nginx alias traversal[1][2], nevertheless
it should only match /static/ and not e.g. /staticfoobar.

[1] d94f18a7 ("Fix nginx alias traversal")
[2] https://github.com/yandex/gixy/blob/641060d6355fbb5bd71695928a2bf14a9bcb8bf2/docs/en/plugins/aliastraversal.md

Fixes: 9294828f ("Setup mailman3 server")

roles/mailman3/templates/nginx.d.conf.j2

@@ -10,8 +10,8 @@ server {
     access_log   /var/log/nginx/{{ lists_domain }}/access.log.json json_main;
     error_log    /var/log/nginx/{{ lists_domain }}/error.log;
 
-    location /static {
-      alias /var/lib/mailman-web/static;
+    location /static/ {
+      alias /var/lib/mailman-web/static/;
     }
 
     # include uwsgi_params