stream: update emit readable debug statement

Currently, the debug statement in emitReadable is `emit readable` which
can be interpreted as the readable event is going to be emitted. But
I think the intent of this debug statment is just that the
emitReadable_ function was entered. If that was not the intent then
perhaps the debug statment should be moved into the if statement below
it.

PR-URL: #22613
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/_stream_readable.js

@@ -529,7 +529,7 @@ function emitReadable(stream) {
 
 function emitReadable_(stream) {
   var state = stream._readableState;
-  debug('emit readable');
+  debug('emitReadable_', state.destroyed, state.length, state.ended);
   if (!state.destroyed && (state.length || state.ended)) {
     stream.emit('readable');
   }