Add missing final dot to pseudo-triple example

Fixes [errata item 25](https://www.w3.org/2001/sw/wiki/RDF1.1_Errata#erratum_25),
[reported by Jodi Schneider](https://lists.w3.org/Archives/Public/public-rdf-comments/2018Apr/0000.html).

spec/index.html

@@ -202,7 +202,7 @@ <h3>Triples</h3>
       &lt;Bob&gt; &lt;is born on&gt; &lt;the 4th of July 1990&gt;.
       &lt;Bob&gt; &lt;is interested in&gt; &lt;the Mona Lisa&gt;.
       &lt;the Mona Lisa&gt; &lt;was created by&gt; &lt;Leonardo da Vinci&gt;.
-      &lt;the video 'La Joconde à Washington'&gt; &lt;is about&gt; &lt;the Mona Lisa&gt;
+      &lt;the video 'La Joconde à Washington'&gt; &lt;is about&gt; &lt;the Mona Lisa&gt;.
     </pre>
 
     <p>The same resource is often referenced in multiple triples. In the example above,