Missing newline before $$ in conventions_notation.md

[stevesimmons]

Nice work Daniel.

I found a typo at line 60 of https://dafriedman97.github.io/mlbook/content/conventions_notation.html, where your second "is written as $$" is missing the empty line before the "$$".

[dafriedman97]

Thanks so much Steve! I went ahead and fixed that. Much appreciated!