fix broken link

Author: pchampin

spec/index.html

@@ -757,7 +757,7 @@ <h3>Literals</h3>
       two, three, or four elements, as follow:</p>
 
     <ol>
-      <li>A <dfn>lexical form</dfn>, being an [=RDF string=].
+      <li>A <dfn>lexical form</dfn>, being an <a data-lt="string">RDF string</a>.
       <li>A <dfn>datatype IRI</dfn>, being an <a>IRI</a>
         identifying a datatype that determines how the lexical form maps
         to a <a>literal value</a>.</li>