Fix typo in TTL check update endpoints

The current website says that you can send output with the update of a check using the `?node` parameter. It wasn't working and I checked [the source](https://github.com/hashicorp/consul/blob/387449837498df43b0ce9b89fea52693ec35addc/command/agent/agent_endpoint.go#L110) to find that it was (the more logical noun of) note, dah!

I changed the indication to replace `?node` for `?note`

Consul's amazing, thanks for releasing this!