Icon warnings in chapter 2

[jonthegeek]

The second code block in 2.2.5 Limited choices throws warnings:

#> This Font Awesome icon ('angry') does not exist:
#> * if providing a custom `html_dependency` these `name` checks can 
#>   be deactivated with `verify_fa = FALSE`
#> This Font Awesome icon ('smile') does not exist:
#> * if providing a custom `html_dependency` these `name` checks can 
#>   be deactivated with `verify_fa = FALSE`
#> This Font Awesome icon ('sad-tear') does not exist:
#> * if providing a custom `html_dependency` these `name` checks can 
#>   be deactivated with `verify_fa = FALSE`

I'd have to dig in to see exactly why that happens, but I'll try to PR a fix when I've had a chance to look into it.

[jonthegeek]

The same warning shows up in 2.2.7, and probably other places where icon() is used.