[Backport 8.14] Update asStream code example (#2244)

https://github.com/elastic/elasticsearch-js/issues/2241 notes that there
is no body attribute on a response. This is mostly just a typo in the
example, as `result` itself is a readable stream, unless `meta: true` is
passed, in which case `result.body` will be a readable stream.

Also dropped the callback-style stream processing example as it's a bit
outdated.

(cherry picked from commit 45e3c0657a)

Co-authored-by: Josh Mock <joshua.mock@elastic.co>

docs/examples/asStream.asciidoc

@@ -57,22 +57,12 @@ async function run () {
     asStream: true
   })
 
-  // stream async iteration, available in Node.js ≥ 10
   let payload = ''
-  body.setEncoding('utf8')
+  result.setEncoding('utf8')
   for await (const chunk of result) {
     payload += chunk
   }
   console.log(JSON.parse(payload))
-
-  // classic stream callback style
-  let payload = ''
-  result.setEncoding('utf8')
-  result.on('data', chunk => { payload += chunk })
-  result.on('error', console.log)
-  result.on('end', () => {
-    console.log(JSON.parse(payload))
-  })
 }
 
 run().catch(console.log)