Update solutions.rstudio.com to align with bike_predict changes

[SamEdwardes]

After #15 has been merged update solutions.rstudio.com to align with the changes:

• website page: https://solutions.rstudio.com/example/bike_predict/
• source code: https://github.com/rstudio/solutions.rstudio.com/blob/main/docs/example/bike_predict.md