Use "read a chunk" operation correctly

The operation takes a reader but we passed a stream. This change fixes that.

Fixes #570.

fetch.bs

@@ -1179,8 +1179,14 @@ or "<code>worker</code>".
  <li><p>If <var>body</var> is null, then <a>queue a fetch task</a> on <var>request</var> to
  <a>process request end-of-body</a> for <var>request</var> and abort these steps.
 
+ <li>
+  <p>Let <var>reader</var> be the result of <a lt="get a reader" for=ReadableStream>getting
+  a reader</a> from <var>body</var>'s <a for=body>stream</a>.
+
+  <p class="note no-backref">This operation cannot throw an exception.
+
  <li><p>Let <var>read</var> be the result of <a lt="read a chunk" for=ReadableStream>reading a
- chunk</a> from <var>body</var>'s <a for=body>stream</a>.
+ chunk</a> from <var>body</var>'s <a for=body>stream</a> with <var>reader</var>.
 
  <li>
   <p><a>In parallel</a>, while true:
@@ -1205,7 +1211,7 @@ or "<code>worker</code>".
       <p class="note no-backref">This step blocks until <var>bs</var> is fully transmitted.
 
      <li><p>Set <var>read</var> to the result of <a lt="read a chunk" for=ReadableStream>reading a
-     chunk</a> from <var>body</var>'s <a for=body>stream</a>.
+     chunk</a> from <var>body</var>'s <a for=body>stream</a> with <var>reader</var>.
     </ol>
 
    <li><p>Otherwise, if <var>read</var> is fulfilled with an object whose <code>done</code> property