fix typo in index.html

Changes the one instance of `a <code>@context` to `an <code>@context`, to match the other six instances of the latter.

Author: TallTed

index.html

@@ -3331,7 +3331,7 @@ <h3>Using the Document Base for the Default Vocabulary</h3>
 
 <section class="informative changed"><h2>Scoped Contexts</h2>
 
-  <p>An <a>expanded term definition</a> can include a <code>@context</code>
+  <p>An <a>expanded term definition</a> can include an <code>@context</code>
     property, which defines a <a>context</a> (a <a>scoped context</a>) for
     <a data-lt="JSON-LD value">values</a> of properties defined using that <a>term</a>.
     When used for a <a>property</a>, this is called a <a>property-scoped context</a>.