doc: broken PerformanceObserver code sample

The code sample at the top of the "Performance measurements API"
section of the docs does not run.

The code in question:

```js
const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});
```

If you replace `doSomeLongRunningProcess` with an IIFE with a sleep()
at the top of it, you get this:

```js
const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');

(async function doSomeLongRunningProcess() {
  await new Promise(r => setTimeout(r, 5000));
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
})()
```

When you run this, you get the following output:

```sh
$ node performance-test.js
17.873416

node:internal/per_context/domexception:53
    ErrorCaptureStackTrace(this);
    ^
DOMException [SyntaxError]: The "A" performance mark has not been set
    at new DOMException (node:internal/per_context/domexception:53:5)
    at __node_internal_ (node:internal/util:695:10)
    at getMark (node:internal/perf/usertiming:65:11)
    at calculateStartDuration (node:internal/perf/usertiming:202:13)
    at measure (node:internal/perf/usertiming:220:7)
    at Performance.measure (node:internal/perf/performance:135:12)
    at /private/tmp/performance-test.js:14:15

Node.js v20.11.1
```

I believe it's due to the call to `performance.clearMarks();` in the
PerformanceObserver callback. If you remove that, it works as expected:

```js
const { PerformanceObserver, performance } = require('node:perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
});
obs.observe({ type: 'measure' });
performance.measure('Start to Now');

performance.mark('A');

(async function doSomeLongRunningProcess() {
  await new Promise(r => setTimeout(r, 5000));
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
})()
```

```sh
$ node performance-test.js
17.761083
5002.468417
```

PR-URL: #54227
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

doc/api/perf_hooks.md

@@ -22,18 +22,18 @@ const { PerformanceObserver, performance } = require('node:perf_hooks');
 
 const obs = new PerformanceObserver((items) => {
   console.log(items.getEntries()[0].duration);
-  performance.clearMarks();
 });
 obs.observe({ type: 'measure' });
 performance.measure('Start to Now');
 
 performance.mark('A');
-doSomeLongRunningProcess(() => {
+(async function doSomeLongRunningProcess() {
+  await new Promise(r => setTimeout(r, 5000));
   performance.measure('A to Now', 'A');
 
   performance.mark('B');
   performance.measure('A to B', 'A', 'B');
-});
+})();
 ```
 
 ## `perf_hooks.performance`