[MLIR][Python][Docs] Fix example of Python-defined dialects (#188174) Some breaking changes are introduced in #186574 for Python-defined dialects. So we need to fix the example in the docs. GitOrigin-RevId: ad39e5a6b84819b75581ab8b1adafd1c8ffc9899
See https://mlir.llvm.org/ for more information.