Renamed the generated html filename for license.

Due to a maven plugin evolution, the former license.html name is now :
licenses.html

src/site/site.xml

@@ -31,7 +31,7 @@
       <item name="Building"                 href="/building.html"                           />
       <item name="Configuration"            href="/configuration.html"                      />
       <item name="FAQ"                      href="/faq.html"                                />
-      <item name="License"                  href="/license.html"                            />
+      <item name="License"                  href="/licenses.html"                            />
       <item name="Downloads"                href="/downloads.html"                          />
       <item name="Changes"                  href="/changes-report.html"                     />
       <item name="Contact"                  href="/contact.html"                            />