tmux-ip: fix DO metadata path (address -> ip_address)

The DigitalOcean metadata endpoint for reserved/floating IPv4 is
/metadata/v1/{reserved,floating}_ip/ipv4/ip_address, not .../address.
The .../active check works but the wrong IP path returned an empty
body, causing the script to silently fall through to ipify on every
droplet with a reserved/floating IP attached.

Verified on a droplet:
  curl .../reserved_ip/ipv4/ip_address -> 45.55.111.240
  curl .../reserved_ip/ipv4/address    -> (empty)

.local/bin/tmux-ip

@@ -19,7 +19,7 @@ do_anchor_ip() {
   for kind in reserved_ip floating_ip; do
     active=$($CURL "$META/$kind/ipv4/active" 2>/dev/null) || continue
     [ "$active" = "true" ] || continue
-    ip=$($CURL "$META/$kind/ipv4/address" 2>/dev/null) || continue
+    ip=$($CURL "$META/$kind/ipv4/ip_address" 2>/dev/null) || continue
     [ -n "$ip" ] && { printf '%s\n' "$ip"; return 0; }
   done
   return 1