Update merchant record

curl --request PATCH \
  --url https://staging-vouchers.ventogram.com/api/v1/merchants \
  --header 'Content-Type: application/json' \
  --header 'X-Api-Key: <api-key>' \
  --header 'X-Api-User: <api-key>' \
  --data '{
  "callbackUrl": "https://www.example.com/webhook/ventogram",
  "logo": "https://www.example.com/favicon",
  "feeBearer": "client",
  "creditCurrency": "NGN"
}'

{
  "success": true,
  "message": "<string>",
  "data": {
    "username": "<string>",
    "callbackUrl": "<string>",
    "logo": "<string>",
    "creditCurrency": "NGN",
    "feeBearer": "<string>"
  }
}