Update vcpkg port

The docopt version used in [vcpkg](https://github.com/microsoft/vcpkg) indicates a 3+ year old version in [vcpkg/ports/vcpkg.json](https://github.com/microsoft/vcpkg/blob/master/ports/docopt/vcpkg.json) ("version-string": "2018-11-01").
Any vcpkg/docopt enthusiasts fancy having a go at an update?