broken link due to missing hyphen in multicheckout.md

[ryenus]

A minor typo in working on [multiple checkout](multicheckout.md):

azure-pipelines-yaml/design/checkout-path.md

Line 4 in d0943c1

While we're still designing and working on [multiple checkout](multicheckout.md), we can make progress on controlling the checkout of `self`.

Where multicheckout.md should be multi-checkout.md, as in https://github.com/microsoft/azure-pipelines-yaml/blob/master/design/multi-checkout.md