Fix help text for runnable plugin command

When creating a plugin without sub commands, the help text included the
command name (kubectl-plugin) instead of the display name (kubectl plugin):

    Usage:
      kubectl-plugin [flags]

The issue is that the usage line for this case does not use the
command path but the raw `Use` string, and this case was not tested.

Add a test for this case and fix UsageLine() to replace the command name
with the display name.

Tested using https://github.com/nirs/kubernetes/tree/sample-cli-plugin-help