Add step for setting up Head DB

I am installing Ops Manager on Windows 2012 Server R2 and found a slight incorrect and then missing step in the docs.

On https://docs.opsmanager.mongodb.com/current/tutorial/manage-filestore-storage/, step 1c was not correct for me. There is no option Snapshot Storage page. Clicking Backup brought me to a page that asks me to set the Head DB directory. Once I do that, only now do I see an option to Configure a File System Store