Fix link in UPGRADES

UPGRADE.rst

@@ -98,9 +98,9 @@ will log a warning on each received request.
 
 To avoid the warning, administrators using a reverse proxy should ensure that
 the reverse proxy sets `X-Forwarded-Proto` header to `https` or `http` to
-indicate the protocol used by the client. See the [reverse proxy
-documentation](docs/reverse_proxy.md), where the example configurations have
-been updated to show how to set this header.
+indicate the protocol used by the client. See the `reverse proxy documentation
+<docs/reverse_proxy.md>`_, where the example configurations have been updated to
+show how to set this header.
 
 (Users of `Caddy <https://caddyserver.com/>`_ are unaffected, since we believe it
 sets `X-Forwarded-Proto` by default.)