Update semantic-notions.tex to add missing parenthesis

[rzach]

Thanks for this. We have a convention to drop outermost parentheses (unless we're specifically interested in the syntax of formulas). Since changing it here would require that we change it in lots of other places, I don't think I'm going to include it. But, the convention wasn't mentioned in the propositional logic part. I've now added it to content/propositional-logic/syntax-and-semantics/formulas.tex