Update regionalisation demo in README (#351)

Author: paddyroddy

README.md

@@ -28,7 +28,7 @@ attrs = [f"PCGDP{year}" for year in range(1950, 2010, 10)]
 w = libpysal.weights.Queen.from_dataframe(mexico)
 mexico["count"], threshold_name, threshold, top_n = 1, "count", 4, 2
 numpy.random.seed(123456)
-model = spopt.MaxPHeuristic(mexico, w, attrs, threshold_name, threshold, top_n)
+model = spopt.region.MaxPHeuristic(mexico, w, attrs, threshold_name, threshold, top_n)
 model.solve()
 mexico["maxp_new"] = model.labels_
 mexico.plot(column="maxp_new", categorical=True, figsize=(12,8), ec="w");