Editorial: fix grammar in notes about run the fullscreen steps

Oversight in #94.

Author: foolip

fullscreen.bs

@@ -300,7 +300,7 @@ these steps:
   </ol>
 
   <p class=note>The order in which elements are <a lt="fullscreen an element">fullscreened</a>
-  is not observable, because the <a>run the fullscreen steps</a> is invoked in <a>tree order</a>.
+  is not observable, because <a>run the fullscreen steps</a> is invoked in <a>tree order</a>.
 
  <li><p>Resolve <var>promise</var> with undefined.
 </ol>
@@ -439,7 +439,7 @@ could be an open <{dialog}> element.
   </ol>
 
  <p class=note>The order in which documents are <a lt="unfullscreen a document">unfullscreened</a>
- is not observable, because the <a>run the fullscreen steps</a> is invoked in <a>tree order</a>.
+ is not observable, because <a>run the fullscreen steps</a> is invoked in <a>tree order</a>.
 
  <li><p>Resolve <var>promise</var> with undefined.
 </ol>