docs: fix returned value for max_headers_count. It was 503 now it is 502. (#40006)

Tested and the actual returned code differs from docs:
```http
❯ curl https://httpbin.example.com/policy/response-headers -ki
HTTP/1.1 502 Bad Gateway
content-length: 87
content-type: text/plain
date: Mon, 23 Jun 2025 17:08:56 GMT
server: envoy

upstream connect error or disconnect/reset before headers. reset reason: protocol error%
```

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Commit Message: fix returned value for ``max_headers_count``. It was 503
now it is 502.
Additional Description:
Risk Level: Low
Testing:
Docs Changes:
https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/protocol.proto#envoy-v3-api-msg-config-core-v3-httpprotocoloptions
Release Notes: fix returned value for ``max_headers_count``. It was 503
now it is 502.
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

---------

Signed-off-by: Jesus Munoz <[email protected]>

Mirrored from https://github.com/envoyproxy/envoy @ 427f3d9aaaebee42c97222a1e75214baeb67723f

Author: update-envoy[bot]

envoy/config/core/v3/protocol.proto

@@ -276,7 +276,7 @@ message HttpProtocolOptions {
   // The default value for responses can be overridden by setting runtime key ``envoy.reloadable_features.max_response_headers_count``.
   // Downstream requests that exceed this limit will receive a 431 response for HTTP/1.x and cause a stream
   // reset for HTTP/2.
-  // Upstream responses that exceed this limit will result in a 503 response.
+  // Upstream responses that exceed this limit will result in a 502 response.
   google.protobuf.UInt32Value max_headers_count = 2 [(validate.rules).uint32 = {gte: 1}];
 
   // The maximum size of response headers.