fix perf example

Author: AbhiPrasad

scenarios/browser/perf-manual.js

@@ -1,12 +1,12 @@
-import { init } from "@sentry/browser";
+import { init, startTransaction } from "@sentry/browser";
 import "@sentry/tracing";
 
 init({
   dsn: "https://[email protected]/0000000",
   tracesSampleRate: 1.0,
 });
 
-const transaction = Sentry.startTransaction({ op: "task", name: "Important Stuff" });
+const transaction = startTransaction({ op: "task", name: "Important Stuff" });
 
 setTimeout(() => {
   transaction.finish();