Update theme.html

[johnmackintosh]

Hi - I found 2 very minor typos on the reference page
https://ggplot2.tidyverse.org/reference/theme.html

This PR updates these typos from "backgronud" to "background" for the entries relating to the theme arguments for
strip.background.x
and
strip.background.y

[dpseidel]

Thanks! These typos have recently been fixed in the source files by tidyverse/ggplot2#2727. The ggplot2 site is built with pkgdown so when typos are found, you should check the source documentation over in the ggplot2 repository and submit PRs there. After changes are merged, periodically the whole site is rebuilt rather than changing the html files directly from the ggplot2-docs repo.