Timeout description does not contain time unit of measure

[guyluz11]

Timeout description does not contain time unit of measure in ping_interface.dart page

dart_ping/dart_ping/lib/src/ping_interface.dart

Line 26 in 9a00369

/// How long to wait for a ping to return before marking it as lost

After checking it on the Linux implementation

dart_ping/dart_ping/lib/src/ping/linux_ping.dart

Line 32 in 9a00369

var params = ['-O', '-n', '-W $timeout', '-i $interval', '-t $ttl'];

I have found that the -W for ping command on Linux is in seconds

-W timeout
Time to wait for a response, in seconds. The option affects only timeout in absence of
any responses, otherwise ping waits for two RTTs. Real number allowed with dot as a
decimal separator (regardless locale setup). 0 means infinite timeout.

I think adding this to our description is important, or change timeout to timeoutInSeconds.