docs: ui.Theme docs missing quotation marks

[randyzwitch]

Reference to pip install shiny[theme] should be pip install "shiny[theme]" . I will submit a PR for this.

https://shiny.posit.co/py/api/core/ui.Theme.html?_gl=1*1ru846s*_ga*MTk3MTM0MDY1Ni4xNzEyNjgzNTY2*_ga_2C0WZ1JHG0*MTcyMjUyMzYxNC4yNDMuMS4xNzIyNTI5OTU5LjAuMC4w