Corrects host.shutdown

[1995parham]

host.shutdown expects HOST as an argument.

[vmwclabot]

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[dougm]

Thanks @1995parham , the USAGE.md doc is actually generated by running make doc. Can you add a Usage method to host/shutdown.go similar to: https://github.com/vmware/govmomi/blob/master/govc/dvs/add.go#L59-L61

[1995parham]

Thanks @dougm for your reply, I add usage method for host shutdown command.

[dougm]

@1995parham thanks!

[vmwclabot]

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[vmwclabot]

@1995parham, VMware has rejected your signed contributor license agreement. The change has already been merged, but it will be backed out by the project maintainers if the agreement is not resigned in a timely manner. Click here to resign the agreement.