Remove root from YAML

Because `make workshop-check` gave

~~~
Superfluous categories: offending entries ['root']
~~~

index.html

 ---
 layout: workshop      # DON'T CHANGE THIS.
-root: .               # DON'T CHANGE THIS EITHER.  (THANK YOU.)
 carpentry: "FIXME"    # what kind of Carpentry (must be either "dc" or "swc")
 venue: "FIXME"        # brief name of host site without address (e.g., "Euphoric State University")
 address: "FIXME"      # full street address of workshop (e.g., "Room A, 123 Forth Street, Blimingen, Euphoria")