Fix README punctuation

This is a minor fix and should satisfy [small patch exception](https://github.com/docker/docker/blob/master/CONTRIBUTING.md#small-patch-exception)

[Guide to semicolon vs dash](http://english.stackexchange.com/a/4015) The README seems to currently favor dashes. Regardless of fix to either dash or semicolon, I think it's clear the status quo should be changed.