the `library(mysterycoffee)` call triggers installation of `mysterycoffee` on CI

Inside the Vignette chapter, an example vignette contains the call to load the (dummy) `mysterycoffee` package. Because the vignette uses R Markdown, the call is inside an R chunk:

````
```{r install}
library(mysterycoffee)
```
````
Even though this chunk is _inside_ a ` ````{verbatim} ` chunk, and renders well on the page, the CI flags the `library()` call as a package that needs to be installed. This does not work, because mysterycoffee is a dummy package.

Options:
- Currently, the call is commented out, and another comment and an instructor note explains why.
- Adding `devtools::install_github("bvreede/mysterycoffee")` (and `install.packages("devtools")`) to the sandpaper main workflow (as done in [#c9672fa](https://github.com/carpentries-incubator/lesson-R-packaging/pull/106/commits/c9672fa0b6cfd9ab096569af122178f3226d0f04) and [#78d7efa](https://github.com/carpentries-incubator/lesson-R-packaging/pull/106/commits/78d7efabaf3bb60c46edce670b9fe5194c144033)) installs a version of mysterycoffee inside the CI, and "rescues" the failing deployment. It is however unnecessary, relies on a dummy package remaining available on someone's github page, and needs to be kept updated if and when the workflow is automatically updated by the Carpentries.
- Ideally, the CI does not flag this line of code as requiring the installation of `mysterycoffee`, but interprets it as what it is, namely, a verbatim bit of example code.