Editorial: use AbortSignal's aborted predicate

Follow-up: integrating abort reason is #1343.

Fixes #1349.

Author: annevk

fetch.bs

@@ -7476,8 +7476,8 @@ method steps are:
  <li><p>Let <var>request</var> be <var>requestObject</var>'s <a for=Request>request</a>.
 
  <li>
-  <p>If <var>requestObject</var>'s <a for=Request>signal</a>'s <a for=AbortSignal>aborted flag</a>
-  is set, then:
+  <p>If <var>requestObject</var>'s <a for=Request>signal</a> is <a for=AbortSignal>aborted</a>,
+  then:
 
   <ol>
    <li><p><a>Abort fetch</a> with <var>p</var>, <var>request</var>, and null.