Fix grammar in Scoped Contexts section.

Fixes #240.

index.html

@@ -3242,8 +3242,8 @@ <h3>Using the Document Base for the Default Vocabulary</h3>
     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>.
-    This allows values to use <a>term definitions</a>, <a>base IRI</a>,
-    <a>vocabulary mapping</a> or <a>default language</a> which is different from the
+    This allows values to use <a>term definitions</a>, the <a>base IRI</a>,
+    <a>vocabulary mappings</a> or the <a>default language</a> which are different from the
     <a>node object</a> they are contained in, as if the
     <a>context</a> was specified within the value itself.</p>