docs(api): correct createPDFStream example (#7538)

Prior to this patch, the example results in `browser.close()` being executed before the file is generated/written to disk. One needs to listen for the `end` event on the `ReadableStream` before closing the browser, otherwise an exception is raised:

    UnhandledPromiseRejectionWarning: Error: Protocol error (IO.read): Target closed

docs/api.md

@@ -1542,7 +1542,9 @@ const puppeteer = require('puppeteer');
   const pdfStream = await page.createPDFStream();
   const writeStream = fs.createWriteStream('test.pdf');
   pdfStream.pipe(writeStream);
-  await browser.close();
+  pdfStream.on('end', async () => {
+    await browser.close();
+  });
 })();
 ```