fix static method without static decorator

Author: Hasenn

graphs/minimum_spanning_tree_boruvka.py

@@ -146,6 +146,7 @@ def union(self, item1, item2):
                 self.parent[root2] = root1
                 return root1
 
+    @staticmethod
     def boruvka_mst(graph):
         """
         Implementation of Boruvka's algorithm