editorial: Do not define Navigator, just reference it (#117)

Navigator is <dfn>ed in HTML; redefining it here does not make much sense as
we just need to reference the original definition.

index.html

@@ -208,8 +208,8 @@ <h2>
         Extensions to the `Navigator` interface
       </h2>
       <p>
-        The [[HTML]] specification defines the <dfn>Navigator</dfn> interface,
-        which this specification extends:
+        The [[HTML]] specification defines the {{Navigator}} interface, which
+        this specification extends:
       </p>
       <pre class="idl">
         [SecureContext, Exposed=(Window)]