Remove (or make optional) semicolons after non-terminals

[osa1]

I don't remember why I required a semicolon after a non-terminal (maybe I just wanted the syntax to be compatible with LALRPOP?), but I think it shouldn't be necessary.

We currently have two syntax for non-terminals:

• With single production:

  <non-terminal name> : <non-terminal type> = <production>,
  

• With multiple productions:

  <non-terminal name> : <non-terminal type> = {
    <production>,*
  };
  

The semicolon in second syntax can be removed.