doc: fix onReadable reentry after unshift called

In example parseHeader, stream listen **readable** event,
if call stream.unshift(buf) before stream.removeListener('readable',
onReadable), readable event will be emited before removeListener,
so callback will not been called correctlly.
After change to
```js
stream.removeListener('error', callback);
stream.removeListener('readable', onReadable);
if (buf.length)
stream.unshift(buf);
```
It solves this problem.

PR-URL: https://github.com/nodejs/node/pull/8200
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/stream.md

@@ -963,10 +963,11 @@ function parseHeader(stream, callback) {
         header += split.shift();
         const remaining = split.join('\n\n');
         const buf = Buffer.from(remaining, 'utf8');
-        if (buf.length)
-          stream.unshift(buf);
         stream.removeListener('error', callback);
+        // set the readable listener before unshifting
         stream.removeListener('readable', onReadable);
+        if (buf.length)
+          stream.unshift(buf);
         // now the body of the message can be read from the stream.
         callback(null, header, stream);
       } else {