Update index.bs

Co-authored-by: Adam Rice <ricea@chromium.org>

Author: youennf

index.bs

@@ -3084,7 +3084,7 @@ The following abstract operations support the implementation of the
  1. Let |startAlgorithm| be an algorithm that returns undefined.
  1. Let |pullAlgorithm| be an algorithm that returns [=a promise resolved with=] undefined.
  1. Let |cancelAlgorithm| be an algorithm that returns [=a promise resolved with=] undefined.
- 1. Let |isTransferring| be true if |underlyingSourceDict|["{{UnderlyingSource/pull}}"] is
+ 1. Let |isTransferring| be true if |underlyingSourceDict|["{{UnderlyingSource/type}}"] is
     "{{ReadableStreamType/transfer}}" and false otherwise.
  1. If |underlyingSourceDict|["{{UnderlyingSource/start}}"] [=map/exists=], then set
     |startAlgorithm| to an algorithm which returns the result of [=invoking=]