A minor defect in help message of kn plugin

[Ying Chun Guo]

See the output of kn hello. Pay attention that kn-hello is used in the usage part and in the last sentence, which we definitely hope to use kn hello instead.

$ kn hello
Longer description of this fantastic plugin that can go over several lines.

Usage:
  kn-hello [command]

Available Commands:
  help        Help about any command
  print       Print the argument
  version     Prints the plugin version

Flags:
  -h, --help   help for kn-hello

Use "kn-hello [command] --help" for more information about a command.

It's an issue related with cobra. I create in our repo to track.