message: fix ref in docs
diff: * [Project Api Docs](https://xeroapi.github.io/xero-node/payroll-au/index.html)
  * [Files Api Docs](https://xeroapi.github.io/xero-node/files/index.html)
  * [Bankfeeds Api Docs](https://xeroapi.github.io/xero-node/bankfeeds/index.html)
  -* [Payroll Docs (NZ)](https://xeroapi.github.io/xero-node/payroll-uk/index.html)
  +* [Payroll Docs (NZ)](https://xeroapi.github.io/xero-node/payroll-nz/index.html)
  * [Payroll Docs (UK)](https://xeroapi.github.io/xero-node/payroll-uk/index.html)
  -* [Payroll Docs (AU)](https://xeroapi.github.io/xero-node/payroll-uk/index.html)
  +* [Payroll Docs (AU)](https://xeroapi.github.io/xero-node/payroll-au/index.html)
  ## Getting Started
  