README: update mailing list url

Old url currently returns 403; modify to url that actually works
with current state of the list server

Signed-off-by: S. Lockwood-Childs <sjl@vctlabs.com>

README

@@ -5152,7 +5152,7 @@ may be rejected, even when they contain important and valuable stuff.
 Please see http://www.denx.de/wiki/U-Boot/Patches for details.
 
 Patches shall be sent to the u-boot mailing list <u-boot@lists.denx.de>;
-see http://lists.denx.de/mailman/listinfo/u-boot
+see https://lists.denx.de/listinfo/u-boot
 
 When you send a patch, please include the following information with
 it: