Fix example usage of ping.size

[stevegrey2]

From latest, the parameter is ping.size. With this, I get ping_rtt_best_seconds, ping_rtt_mean_seconds etc. metrics, with the existing example using ping.payload-size all I get is ping_loss_ratio.

docker exec -it ping_exporter /bin/sh
/app # ./ping_exporter --help
usage: ping_exporter [<flags>] [<targets>...]

Flags:
  --help                        Show context-sensitive help (also try --help-long and --help-man).
  --version                     Print version information
  --web.listen-address=":9427"  Address on which to expose metrics and web interface
  --web.telemetry-path="/metrics"  
                                Path under which to expose metrics
  --config.path=""              Path to config file
  --ping.interval=5s            Interval for ICMP echo requests
  --ping.timeout=4s             Timeout for ICMP echo request
  --ping.size=56                Payload size for ICMP echo requests
  --ping.history-size=10        Number of results to remember per target
  --dns.refresh=1m              Interval for refreshing DNS records and updating targets accordingly (0 if disabled)
  --dns.nameserver=""           DNS server used to resolve hostname of targets
  --options.disable-ipv6        Disable DNS from resolving IPv6 AAAA records
  --log.level="info"            Only log messages with the given severity or above. Valid levels: [debug, info, warn, error,
                                fatal]
  --metrics.deprecated="disable"  
                                Enable or disable deprecated metrics (`ping_rtt_ms{type=best|worst|mean|std_dev}`). Valid
                                choices: [enable, disable]
  --metrics.rttunit="s"         Export ping results as either seconds (default), or milliseconds (deprecated), or both (for
                                migrations). Valid choices: [s, ms, both]

Args:
  [<targets>]  A list of targets to ping

[czerwonk]

Thanks for contributing! 👍