Fix local tarsnap key path in readme

Turns out it should be in the tarsnap role, not common. :)

README.textile

 
 Create a new machine key for your server:
 
-bc. tarsnap-keygen --keyfile roles/common/files/root_tarsnap.key --user me@example.com --machine example.com
+bc. tarsnap-keygen --keyfile roles/tarsnap/files/root_tarsnap.key --user me@example.com --machine example.com
 
 h3. 3. Prep the server