Fix wrong link for https_proxy in lowercase explaination

Author: Nek-

setup/symfony_server.rst

@@ -530,5 +530,5 @@ help debug any issues.
 .. _`Proxy settings in Windows`: https://www.dummies.com/computers/operating-systems/windows-10/how-to-set-up-a-proxy-in-windows-10/
 .. _`Proxy settings in macOS`: https://support.apple.com/guide/mac-help/enter-proxy-server-settings-on-mac-mchlp2591/mac
 .. _`Proxy settings in Ubuntu`: https://help.ubuntu.com/stable/ubuntu-help/net-proxy.html.en
-.. _`is treated differently`: https://ec.haxx.se/usingcurl/usingcurl-proxies#http_proxy-in-lower-case-only
+.. _`is treated differently`: https://superuser.com/a/1799209
 .. _`Docker compose CLI env var reference`: https://docs.docker.com/compose/reference/envvars/