You are correct, the way it is written in the title is wrong. One possible notation as I was taught many years ago would be to add an "R" for Ramanujan above the equals sign to point out that we're talking about the sum of a diverging infinite series.

No idea if it's the correct notation, especially since I've seen different versions of it.

That said, there are multiple proofs that the Riemann zeta function of -1, which is the infinite series mentioned above, does in fact equal -1/12.

As a reminder, the Riemann zeta function is this:

zeta(s) = sum(n=1,inf) 1/n^s

If you restrict it to real numbers, it converges for all s>1, so by evaluating it at s=-1, you are messing around with a diverging series, which is why it looks so funky. It's a holomorphic continuation outside its defined area and it seems to work for theoretical physics. And it gives me a headache.