Reference to "bonus instructions" needs to be removed from RStudio lab

This line references "bonus instructions"

intro_to_r/modules/RStudio/lab/RStudio_Lab.Rmd

Line 115 in f05c3d3

    
           Add a new R chunk below the bonus instructions.You can use the insert chunk button or copy paste an existing code chunk. Include a lowercase x inside the chunk as the code. Thus it should look like the chunk above but without `<- c(1,2,3)`. Make sure you press the knit button after this to see what the new chunk looks like.

People were confused since we changed the text to be "Practice on your own"