Missing "-u" option in growpart man

[minWi]

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

[Paride Legovini (paride)]

Bug report moved to GitHub: https://github.com/canonical/cloud-utils/issues/43