Place the following in the server block of your sites nginx config, ie /etc/nginx/sites-available/example.com
# Multisite
# https://github.com/roots/trellis/blob/master/roles/wordpress-setup/templates/wordpress-site.conf.j2
if (!-e $request_filename) {
rewrite /wp-admin$ $scheme://$host$uri/ permanent;
rewrite ^(/[^/]+)?(/wp-.*) /wp$2 last;
rewrite ^(/[^/]+)?(/.*\.php) /wp$2 last;
}