update the vignette

[KKulma]

In the official vignette, the suggested way of referring to code chunks ( {r, include = FALSE, demo = "how_to_pipe"} ) returns errors. However, if we simply name the code chunk 'how_to_pipe': {r how_to_pipe, include = FALSE}, the examples work. I can see that the Github repo's vignette reflects this but not the one published under the above URL.