[docs] use correct json mime type in example

[aknoerig]

If it's not application/json, some servers will not parse it properly into json (as in my case, the nodejs express.json() middleware)

Checklist

Not all of these might apply to your change but the more you are able to check the easier it will be to get your contribution merged.

• [x] CI passes
• [x] Description of proposed change
• [x] Documentation (README, code doc blocks, etc) is updated
• [ ] Existing issue is referenced if there is one
• [ ] Unit tests for the proposed change