stream: fix comment for sync flag of ReadableState

It seems that the comment for sync flag of ReadableState is
copied from WritableState without modification.

PR-URL: #11139
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>

Author: Wang Xinyong

lib/_stream_readable.js

@@ -63,10 +63,10 @@ function ReadableState(options, stream) {
   this.endEmitted = false;
   this.reading = false;
 
-  // a flag to be able to tell if the onwrite cb is called immediately,
-  // or on a later tick.  We set this to true at first, because any
-  // actions that shouldn't happen until "later" should generally also
-  // not happen before the first write call.
+  // a flag to be able to tell if the event 'readable'/'data' is emitted
+  // immediately, or on a later tick.  We set this to true at first, because
+  // any actions that shouldn't happen until "later" should generally also
+  // not happen before the first read call.
   this.sync = true;
 
   // whenever we return null, then we set a flag to say