Copenhagen: Improve documentation on the use of backticks

[JoaodcPires]

For the code

# Create nice labels for case definition:
caselabs <- ggplot2::as_labeller(c(`FALSE` = "Non-case", 
                                   `TRUE` = "Case"))

Adding the notes on the use of backticks:

#Enclosing variable/names in ``
# this is necessary for illegal names. TRUE and FALSE are part of the R syntax.
# In order to select columns or variables that have illegal names, we need to
# enclose them in backticks

caselabs <- ggplot2::as_labeller(c(`FALSE` = "Non-case", 
                                   `TRUE` = "Case"))