Fix: add ref

Signed-off-by: janepie <49834966+janepie@users.noreply.github.com>

Author: janepie

admin_manual/installation/harden_server.rst

@@ -250,6 +250,8 @@ security headers are shipped.
 .. _Web TLS Profiler: https://tlsprofiler.danielfett.de/
 .. _RFC 4086 ("Randomness Requirements for Security"): https://tools.ietf.org/html/rfc4086#section-5.2
 
+.. _connections_to_remote_servers:
+
 Connections to remote servers
 -----------------------------