You may be able to sue, but it's probably not worth doing so. While it may well have been negligent, or unreasonably careless, to have missed the broken toe, all you can recover is an amount of money equivalent to the extra or additional medical costs incurred due to the delay in diagnosis, and posssibly some "pain and suffering" if the delay caused significant, long-lasting impairment. But to get that amount of money, which for a broken toe is not likely to be large, you'd have to hire a medical expert (a doctor) to testify, and such experts do not work cheap--you could easily spend more on the lawsuit than you'd get out of it.