Updated HOL.md to use 'install-windowsFeature' instead of 'add-'

@line 250, updated from 'add-windowsFeature' to 'install-windowsFeature'
From install-windowsFeature help:
This cmdlet is equivalent to and replaces Add-WindowsFeature, the cmdlet that was used to install roles, role services, and features in WindowsServer2008R2
Merge pull request #8 from jschell/patch-1

Thanks for the update!