broken visibility links in README

[bhogan-mitre]

The README links referring to visibility are broken. The URL points to directory /_tests_/, where it should be /__tests__/.

For example:
[visibility rules](https://github.com/plotly/react-chart-editor/tree/master/src/components/containers/_tests_/ConnectedContainersVisibility-test.js)

[nicolaskruchten]

Thanks, I'll fix this shortly!