[mkdocs] Update link in Edit this Page button

[daexs]

An 'Edit this Page' button has been added to the example pages, however, the link it opens is https://github.com/daexs/plotly.js/edit/mkdocs-conversion/pages/... (which also doesn't work due to recent refactoring), we would need to change it to the plotly/plotly.js repo's corresponding branch.