narrow build status badge branch
<td>Build Status</td>
<td>
<a href="https://github.com/bokeh/bokeh/actions">
- <img src="https://github.com/bokeh/bokeh/workflows/GitHub-CI/badge.svg"
+ <img src="https://github.com/bokeh/bokeh/workflows/GitHub-CI/badge.svg?branch=master"
alt="Current github actions build status" />
</a>
</td>
