docs(js): Use arrow function in some code samples (#4523)

Using `=>` to create a function in the sample code.

Author: alifarooq0

src/platforms/node/guides/koa/index.mdx

@@ -31,8 +31,8 @@ const app = new Koa();
 Sentry.init({ dsn: "___PUBLIC_DSN___" });
 
 app.on("error", (err, ctx) => {
-  Sentry.withScope(function(scope) {
-    scope.addEventProcessor(function(event) {
+  Sentry.withScope((scope) => {
+    scope.addEventProcessor((event) => {
       return Sentry.Handlers.parseRequest(event, ctx.request);
     });
     Sentry.captureException(err);