Rename payload to bridgeService

[baubakg]

The payload in the bridgeservice demo documentation in the with-bridge-service branch is incorrect.

Should be called:

curl --request POST \
  --url http://localhost:8080/call \
  --header 'Content-Type: application/json' \
  --data '{
	"callContent": {
          "checkServerState": {
            "class": "com.bridgeservice.demo.backend.MyBlogBackEnd", 
            "method": "isSystemUp", 
            "args": ["http://localhost:8000/"]
          }
        }
}'