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")