lib: convert to arrow function

PR-URL: #24596
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: HisadaNaojiro

lib/internal/socket_list.js

@@ -47,7 +47,7 @@ class SocketListSend extends EventEmitter {
     this._request({
       cmd: 'NODE_SOCKET_GET_COUNT',
       key: this.key
-    }, 'NODE_SOCKET_COUNT', false, function(err, msg) {
+    }, 'NODE_SOCKET_COUNT', false, (err, msg) => {
       if (err) return callback(err);
       callback(null, msg.count);
     });