Improve --help output

[simonswine]

Currently the CLI is providing a somewhat confusing help output:

      --pulsar.url=""            The URL of the remote Pulsar server to send samples to. Example: pulsar://pulsar-proxy:6650 or pulsar+ssl://pulsar-proxy:6651. None, if empty.

But if you actually supply it with quotes it doesn't work:

$ ./prometheus-pulsar-remote-write-linux-amd64 '--pulsar.url="pulsar+ssl://pulsar-proxy:6651"'
level=error ts=2020-08-07T07:52:12.640Z caller=main.go:265 msg="Failed to initialize Pulsar" err="Invalid service URL: InvalidConfiguration"

$ ./prometheus-pulsar-remote-write-linux-amd64 --pulsar.url '"pulsar+ssl://pulsar-proxy:6651"'
ERRO[0000] Failed to parse service URL                   error="parse \"\\\"pulsar+ssl://pulsar-proxy:6651\\\"\": first path segment in URL cannot contain colon"

What works is:

$ ./prometheus-pulsar-remote-write-linux-amd64 --pulsar.url="pulsar+ssl://pulsar-proxy:6651"
level=info ts=2020-08-07T07:52:18.509Z caller=main.go:307 msg="Starting up..."

$ ./prometheus-pulsar-remote-write-linux-amd64 '--pulsar.url=pulsar+ssl://pulsar-proxy:6651'
level=info ts=2020-08-07T07:52:29.481Z caller=main.go:307 msg="Starting up..."

$ ./prometheus-pulsar-remote-write-linux-amd64 --pulsar.url "pulsar+ssl://pulsar-proxy:6651"
level=info ts=2020-08-07T07:55:20.306Z caller=main.go:307 msg="Starting up..."