cdk docs: doesn't work on cloud9

[eladb]

Since there is no browser, cdk docs errors with:

$ cdk docs
(node:8829) UnhandledPromiseRejectionWarning: Error: Command failed: open /home/ec2-user/.nvm/versions/node/v8.11.0/lib/node_modules/aws-cdk-docs/dist/docs/index.html
Couldn't get a file descriptor referring to the console

    at ChildProcess.exithandler (child_process.js:275:12)
    at emitTwo (events.js:126:13)
    at ChildProcess.emit (events.js:214:7)
    at maybeClose (internal/child_process.js:925:16)
    at Socket.stream.socket.on (internal/child_process.js:346:11)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at Pipe._handle.close [as _onclose] (net.js:567:12)
(node:8829) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:8829) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
Unknown command: docs

If it is impossible to open a browser, we should at least tell the users how they can open the docs themselves...