Update -addr flag help usage

[palourde]

A Sensu user reported some difficulties while trying to configure the -addr flag of this influx handler.

After digging into the influx client code, it seems like a specific format is required: https://github.com/influxdata/influxdb/blob/badc1d8ed3056e46f0855223c500152eae0c5f1f/client/v2/client.go#L25

This PR simply updates the help usage for the -addr flag.

I also updated the documentation: https://github.com/sensu/sensu-docs/pull/692

[nikkictl]

@palourde thanks for doing this! Would you mind updating the example handler definition in the readme as well?

[palourde]

@nikkiki Done!