Update placeholder date in 2.0 release notes
2.0.0
=====
-Bokeh Version ``2.0.0`` (XXXX 2019) is a major milestone of the Bokeh project.
+Bokeh Version ``2.0.0`` (March 2020) is a major milestone of the Bokeh project.
This release cycle closed more than 180 issues. Highlights include:
* New, modern ``MultiChoice`` widget added :bokeh-issue:`9532`
