Update a with an

Co-authored-by: François Beaufort <[email protected]>

Author: zolkis

index.html

@@ -3466,7 +3466,7 @@ <h3><dfn>Writing content</dfn></h3>
               </li>
               <li>
                 If |reader| is already in the <a>activated reader objects</a>,
-                then reject |p| with a {{"InvalidStateError"}} {{DOMException}}
+                then reject |p| with an {{"InvalidStateError"}} {{DOMException}}
                 and abort these steps.
               </li>
               <li>