docs: fix table formatting.
Command-line options are:
| Option | Type | Default value | Description |
-| -------------- | ------ | ------------------------ |
+| -------------- | ------ | ------------------------ | ------------------------------------ |
| `volume-group` | string | "" | target volume group name |
| `listen` | string | `/run/topolvm/lvmd.sock` | unix domain socket endpoint of gRPC |
| `spare` | uint64 | 10 | Storage capacity in GiB to be spared |
