Fixes update extension github link.

extensions/firefox/update.rdf

@@ -15,7 +15,7 @@
                 <em:minVersion>6.0</em:minVersion>
                 <em:maxVersion>11.*</em:maxVersion>
                 <!-- Use the raw link for updates so we we can use SSL. -->
-                <em:updateLink>https://github.com/mozilla/pdf.js/raw/gh-pages/extensions/firefox/pdf.js.xpi</em:updateLink>
+                <em:updateLink>https://raw.githubusercontent.com/mozilla/pdf.js/gh-pages/extensions/firefox/pdf.js.xpi</em:updateLink>
               </RDF:Description>
             </em:targetApplication>
           </RDF:Description>