Fix http_error_message_format buckconfig documentation
Fix http_error_message_format buckconfig documentation

Summary:
`concept/buckconfig.html` is broken.

Fixing the http_error_message_format buckconfig option to make page working again.

Test Plan:
```
./docs/soyweb-local.sh
```

Reviewed By: marcinkosiba

fbshipit-source-id: 070f068

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