Fix a mismatched comment & implementation in CircuitTemplate.update_var()

[PC-DOS]

The comment in CircuitTemplate.update_var() says elements in the edge_vars list are tuple with 4 elements ((source, target, idx, edge_dict)), but the real implementation left idx in the for expression, causing problems.

Old expression:

        for source, target, edge_dict in edge_vars:
            _, _, _, base_dict = self.get_edge(source, target)
            base_dict.update(edge_dict)

Updated expression:

        for source, target, idx, edge_dict in edge_vars:
            _, _, _, base_dict = self.get_edge(source, target, idx)
            base_dict.update(edge_dict)