remove bookdown as suggested dependency

DESCRIPTION

@@ -70,7 +70,6 @@ Suggests:
     epiparameter,
     fitdistrplus,
     knitr,
-    bookdown,
     rmarkdown,
     ggplot2,
     spelling,

inst/WORDLIST

@@ -3,7 +3,6 @@ Althaus
 Barré
 Boëlle
 Boesmans
-bookdown
 bpmodels
 bw
 Camacho

vignettes/design-principles.Rmd

@@ -52,7 +52,7 @@ The aim is to restrict the number of dependencies to a minimal required set for
 
 {stats} is distributed with the R language so is viewed as a lightweight dependency, that should already be installed on a user's machine if they have R. {checkmate} is an input checking package widely used across Epiverse-TRACE packages.  {bpmodels} is used to simulate a single-type branching process model to calculate the probability of containment (`probability_contain()`), as mentioned above, branching process simulations are beyond the scope of {superspreading}.
 
-Suggested dependencies (not including package documentation ({knitr}, {bookdown}, {rmarkdown}), testing ({spelling} and {testthat}), and plotting ({ggplot2})) are: [{epiparameter}](https://github.com/epiverse-trace/epiparameter), used to easily access epidemiological parameters from the package's library, and [{fitdistrplus}](https://CRAN.R-project.org/package=fitdistrplus), used for model fitting methods.
+Suggested dependencies (not including package documentation ({knitr}, {rmarkdown}), testing ({spelling} and {testthat}), and plotting ({ggplot2})) are: [{epiparameter}](https://github.com/epiverse-trace/epiparameter), used to easily access epidemiological parameters from the package's library, and [{fitdistrplus}](https://CRAN.R-project.org/package=fitdistrplus), used for model fitting methods.
 
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