Update README.md to reflect proper syntax for docker container

[bschonec]

This might not be the ultimate solution for why the docker container doesn't run properly; I'm not a developer.

Someone smarter than me can probably find the ultimate fix but changing the documentation so that the linter works is a short-term solution for guys like me that just want to check their puppet code.

[CLAassistant]

All committers have signed the CLA.

[bschonec]

I realize this change doesn't fix the code but it would be very helpful to document the behavior properly until the code can be fixed. A merge would be most appreciated.

[eputnam]

my apologies, i'm not sure why i thought you could merge.