replace link to Dash in sidebar

[Coding-with-Adam]

hi @LiamConnors
@jingningzhang1 and I were talking last week about the side bar in the plotly GO docs. The Dash link under the quick reference section leads to Dash Enterprise. We believe it would be better that the link lead to the new Dash in 20 tutorial given its open source nature and the fact that it's a page that will help people learn Dash (which is probably why most people click that link).