Change final exercises in R markdown lesson to be in Exercise box

[Zena Lapp]

e.g.:

> ## Exercise: R Markdown syntax
> Go ahead and do some online searches on how to do the following:
> * create a bullet point list with three items
> * as the first item, write the name of your currently favorite programming language in bold
> * as the second item, write  the name of a function you have so far found most useful in italics
> * as the third item, write one thing you want to learn next on your programming journey in bold and italics
> * turn your bullet point list into a numbered list
> * create a fourth list item and find an online guide and/or cheat sheet for basic Markdown syntax, write its name down here and hyperlink its url
>
> > ## Solution
> > [This link](https://rmarkdown.rstudio.com/authoring_basics.html) has some helpful basic R Markdown syntax.
> {: .solution}
{: .challenge}

Could have sets of exercises with several solutions instead of having each one be its own exercise box.