It’s indeed a myth.
According to it, I’ve heard anywhere from 3–8 spiders swallowed per year, which would be 0.008–0.02 spiders on a typical night (in a non leap year). But that value varies depending on many factors like location, season, conditions, temperatures, humidity, etc.
If you don’t believe in the myth, the chance of it happening on any night is almost none, but that doesn’t mean it’s impossible.