doxygen/footer.html: Add missing closing tags and don't open a new tab.

The footer template from Doxygen has the closing </body> </html>
as Doxygen doesn't add them otherwise.

target="_blank" was omitted as it's not useful here but
it can be slightly annoying as one cannot just go back
in the browser history.

Since the footer links to the license file in the same
directory and not to CC website, the rel attributes
can be omitted.

doxygen/footer.html

@@ -3,9 +3,11 @@
 <p style="text-align: right;padding-right: 12px;">
 	XZ logo &copy; 2023 by Jia Tan is licensed under
 	<a href="COPYING.CC-BY-SA-4.0"
-	   target="_blank"
-	   rel="license noopener noreferrer"
+	   rel="license"
 	   style="display:inline-block;">
 		CC BY-SA 4.0
 	</a>
 </p>
+
+</body>
+</html>