Fix openapi.yaml for /wallet/updateChangeAddress

[kushti]

it should be {"address": "..."} json, not just pk, e.g.

curl -X 'POST' \
  'http://127.0.0.1:9053/wallet/updateChangeAddress' \
  -H 'accept: application/json' \
  -H 'api_key: hello' \
  -H 'Content-Type: application/json' \
  -d '{
"address":"9fPRvaMYzBPotu6NGvZn4A6N4J2jDmRGs4Zwc9UhFFeSXgRJ8pS"
}'