change suggested cipher for nginx

docs/Running-Mastodon/Production-guide.md

@@ -24,7 +24,7 @@ server {
 
   ssl_protocols TLSv1.2;
   ssl_ciphers EECDH+AESGCM:EECDH+AES;
-  ssl_ecdh_curve secp384r1;
+  ssl_ecdh_curve prime256v1;
   ssl_prefer_server_ciphers on;
   ssl_session_cache shared:SSL:10m;