[go-server] fix addResponseHeaders tpl option typo

[mgoltzsche]

According to the documentation the go-server generator should support an addResponseHeaders option but a template uses an addResponseHeader option in some places which this PR fixes.

Closes #9795

@antihax @grokify @kemokemo @jirikuncar @ph4r5h4d

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[wing328]

Good catch. The fix looks good to me 👍

Thanks for the contribution.