What's wrong with this picture?
You might think it's the absence of an apostrophe in Bakers. And that would be a valid thought. An argument can be made for adding an apostrophe, either before or after the s. But an argument can also be made for leaving the apostrophe out.
You already know that Baker's Square would refer to a square belonging to one baker and that Bakers' Square would refer to a square belonging to more than one. It's possible one of those meanings is intended (although I'm not sure because I don't understand the origin of the term Bakers Square--what exactly is the square?).
I think it's more likely, though, that the meaning is a square (as in an area, like a town square) for bakers, not belonging to them. In this case, no apostrophe is needed. You're not talking about possession. It's like Veterans Day or Diners Club--it's a day for veterans, a club for diners. This is called the attributive form.
BUT ... this is not what I found wrong with the sign. (A long digression, I know--consider this a two-fer.) My gripe is the unparallelism of the description of the place. RESTAURANT & PIES. So you're a restaurant and ... pies? Or you sell pies and ... a restaurant? Nuh-uh. Something wrong with FOOD & PIES or RESTAURANT & PIE BAKERY? Nope. So please change your sign accordingly, BS. And, yes, I will accept a free piece of French silk pie as a token of your gratitude!
Stay tuned for more on parallelism ...