minor #13360 Change code example for custom header when using a reverse proxy (akondas)

This PR was submitted for the 5.0 branch but it was merged into the 3.4 branch instead (closes #13360).

Discussion
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Change code example for custom header when using a reverse proxy

I know this is a very subtle change, but it took me some time to catch the small difference

Commits
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2089a48 Change code example for custom header when using a reverse proxy

Author: javiereguiluz

deployment/proxies.rst

@@ -95,7 +95,7 @@ In this case, you'll need to set the header ``X-Forwarded-Proto`` with the value
     // web/app.php
 
     // ...
-    $_SERVER['HEADER_X_FORWARDED_PROTO'] = $_SERVER['HEADER_CUSTOM_FORWARDED_PROTO'];
+    $_SERVER['HTTP_X_FORWARDED_PROTO'] = $_SERVER['HTTP_CUSTOM_FORWARDED_PROTO'];
     // ...
     $response = $kernel->handle($request);