Fix XHTML Doxygen docs inconsistencies

doc/footer.html

@@ -1,6 +1,7 @@
         </div>
         <div id="footer-container">
             <div id="footer">
+                CSFML is licensed under the terms and conditions of the <a href="https://www.sfml-dev.org/license.php">zlib/png license</a>.<br />
                 Copyright &copy; Laurent Gomila &nbsp;::&nbsp;
                 Documentation generated by <a href="https://www.doxygen.nl/" title="doxygen website">doxygen</a> &nbsp;::&nbsp;
             </div>