[rust-server] Update templating docs to correct the name of x-response-id

[mattdowdell]

I spotted a way to control the enum variants in rust-server for the responses in the docs, tried it out and discovered it doesn't work. Further digging found that the property was simply mislabelled in the docs and works as expected when using the correct property name.

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[wing328]

FYI. @richardwhiuk

[wing328]

@mattdowdell thanks for the PR. Confirmed it's x-response-id in the code (java class)