[bug] Suboptimal number formatting for trendline coefficients in plotly.express

In https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/plotly/express/_core.py#L239 we should use `%g` instead of `%f` so that enough significant digits are displayed even for very small numbers (for the slope in particular). Raised by https://community.plot.ly/t/how-to-extend-ols-trendline/32270