Add missing "a" and tweak nearby whitespace

source

@@ -101528,14 +101528,17 @@ interface <dfn>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span> {
      fetch</span> steps with <var>response</var>.</p></li>
     </ol>
 
-    <p>If the algorithm asynchronously completes with null or with <var>script</var> whose <span
+    <p>If the algorithm asynchronously completes with null or with a <var>script</var> whose <span
     data-x="concept-script-error-to-rethrow">error to rethrow</span> is non-null, then:</p>
+
     <ol>
-    <li><p><span>Queue a task</span> to <span data-x="concept-event-fire">fire an event</span> named
-    <code data-x="event-error">error</code> at <var>worker</var>.</p></li>
-    <li><p>Run the <span data-x="environment discarding steps">environment discarding steps</span>
-    for <var>inside settings</var>.</p></li>
-    <li><p>Return.</p></li>
+     <li><p><span>Queue a task</span> to <span data-x="concept-event-fire">fire an event</span>
+      named <code data-x="event-error">error</code> at <var>worker</var>.</p></li>
+
+     <li><p>Run the <span data-x="environment discarding steps">environment discarding steps</span>
+     for <var>inside settings</var>.</p></li>
+
+     <li><p>Return.</p></li>
     </ol>
 
     <p>Otherwise, continue the rest of these steps after the algorithm's asynchronous completion,