fix: typos in emqx_schema_i18n.conf

apps/emqx/i18n/emqx_schema_i18n.conf

@@ -2005,7 +2005,7 @@ How often to send PING frames to keep a connection alive. 0 means disabled.
     }
     label: {
         en: "Keep Alive Interval"
-        zh: "PING保活频率"
+        zh: "PING 保活频率"
     }
 }