fix broken links in documentation

Hi, this is a small pull request to fix a couple broken links in the Druid documentation. 

The docs point to the capitalized version of the links, but the file name in the docs are lowercase for the "vs redshift" and "vs vertica" docs. This is causing the links to these files to be broken (for example, at http://druid.io/docs/0.6.10/)

Thanks!
Haj