No worries, should be quite easy to rectify. If you're using the iD editor, you can right-click on a node that the building and the power line share and select "Disconnect" from the popup menu (alternatively, left-click on a node to select it, and then press D). Once this is done, there will be 2 nodes (one for the building and another one for the power line), and you'll need to delete the power line one (since it wasn't there before). Then repeat this for all the shared nodes.