doc: fix mistake in http2stream.respondWithFile.

http2stream.respondWithFile api has changed since 8.5.0 with the
addition of the onError option. In the first code example an
onError function is implemented but never used, fix this
mistake.

Add a description to have more informations when onError is triggered.

Fixes: nodejs#15390

doc/api/http2.md

@@ -1147,7 +1147,8 @@ of the given file:
 
 If an error occurs while attempting to read the file data, the `Http2Stream`
 will be closed using an `RST_STREAM` frame using the standard `INTERNAL_ERROR`
-code.
+code. If the `onError` callback is defined it will be called, otherwise
+the stream will be destroyed.
 
 Example using a file path:
 
@@ -1170,7 +1171,7 @@ server.on('stream', (stream) => {
 
   stream.respondWithFile('/some/file',
                          { 'content-type': 'text/plain' },
-                         { statCheck });
+                         { statCheck, onError });
 });
 ```