Correct the rookflex usage information
Correct the rookflex usage information

As the `rookflex` volume plugin, the root command should be
`rookflex` instead of `rook`, else we'll get below confusing
message:

```
...bin/#./rookflex help
Rook Flex volume plugin

Usage:
  rook [command]

Available Commands:
  help        Help about any command
  init        Initialize the volume plugin
  mount       Mounts the volume to the pod volume
  unmount     Unmounts the pod volume
```

Signed-off-by: Dennis Chen <c6bddc0d12324a73dba2bceeae9d65d4fa2c2e25@arm.com>

rook/rook,rook/rook,rook/nfs,rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook