template: reword
template: remove 'category'

I'd say that:
- placing a snippet in a directory should provide enough categorization
- if we really wanted to write the category in the file, it should indeed
  be one of a given list.  But this could be done effectively only with
  a web interface, which we won't have (probably for a long time); right
  now asking contributors to look up a category from a given list would
  be just a nuisance.
  Anyway, we could use a script to add this later based on the directory.