Fix typo in category template creation

[annezazu]

This came up as part of the fifteenth call for testing for the FSE Outreach Program which explores using the category templates:

The description for the category template would be clearer to me with an ‘a’ added: “Displays latest posts in a single post category.”

Right now, this is what appears:

Compared to other categories, it's clear that "a" is added in. I can't see a clear reason why this is left out for categories cc @ntsekouras in case I'm missing something or if this can be a quick update :)