docs(deployment): uncomment XFF in nginx example

Keep X-Forwarded-For as an active default so the snippet is
copy-paste safe; TRUSTED_PROXY_COUNT=1 relies on the proxy
rewriting XFF, and a commented-out line is easy to miss.

docs/DEPLOYMENT.md

@@ -83,7 +83,7 @@ server {
         proxy_pass http://127.0.0.1:3000;
 
         # ⚠️ 反代必须覆写 XFF——见下方「⚠️ 反代必须覆写 XFF」说明。
-        # proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
         proxy_set_header Host $host;