Update README.md
"flat" style PyPI badge from shields.io is no longer working without style explicitly provided
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<a href="https://docs.bokeh.org/en/latest/docs/installation.html">
- <img src="https://img.shields.io/pypi/dm/bokeh?label=PyPI"
+ <img src="https://img.shields.io/pypi/dm/bokeh?label=PyPI&style=flat"
alt="PyPI downloads per month" />
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