[LP: #1710233] Missing "-u" option in growpart man

Originally reported by: [minWi](https://launchpad.net/~e-minguez) on 2017-08-11
Original bug URL: https://bugs.launchpad.net/cloud-utils/+bug/1710233
Last Launchpad Status: Confirmed
Last Launchpad Priority: Low

---

Even if the usage[1] says:

```
-u | --update R update the the kernel partition table info after growing
this requires kernel support and 'partx --update'
R is one of:
- 'auto' : [default] update partition if possible
- 'force' : try despite sanity checks (fail on failure)
- 'off' : do not attempt
- 'on' : fail if sanity checks indicate no support
```

Man doesn't have that flag documented[2].

[1] http://bazaar.launchpad.net/~cloud-utils-dev/cloud-utils/trunk/view/head:/bin/growpart#L121
[2] http://bazaar.launchpad.net/~cloud-utils-dev/cloud-utils/trunk/view/head:/man/growpart.1