docs: change model in vignette to avoid warnings

DESCRIPTION

@@ -42,7 +42,6 @@ Suggests:
     knitr,
     markdown,
     testthat (>= 3.0.0),
-    MASS,
     ciTools
 VignetteBuilder: knitr
 Config/testthat/edition: 3

vignettes/incidence2.Rmd

@@ -9,7 +9,7 @@ vignette: >
   %\VignetteIndexEntry{An introduction to incidence2}
   %\VignetteEngine{knitr::knitr}
   %\VignetteEncoding{UTF-8}
-  %\VignetteDepends{outbreaks, ggplot2, MASS, ciTools, tidyr}
+  %\VignetteDepends{outbreaks, ggplot2, ciTools, tidyr}
 ---
 
 ```{r, include = FALSE}
@@ -545,18 +545,23 @@ When called on incidence2 objects, these methods will utilise the
 underlying grouping structure without the user needing to explicitly state
 what it is. This makes it very easy to utilise in analysis pipelines.
 
-#### Example fitting a negative binomial model
+#### Example fitting a poisson model
 
 ```{r}
 # first twenty weeks of the ebola data set across hospitals
 dat <- incidence_(ebola, date_of_onset, groups = hospital, interval = "isoweek")
 dat <- keep_first(dat, 20L)
 
-# fit a negative binomial to the grouped data
+# fit a poisson model to the grouped data
 (fitted <-
     dat |>
     nest(.key = "data") |>
-    mutate(model  = lapply(data, \(x) MASS::glm.nb(count ~ date_index, x))))
+    mutate(
+        model  = lapply(
+            data,
+            function(x) glm(count ~ date_index, data = x, family = poisson)
+        )
+    ))
 # Add confidence intervals to the result
 (intervals <-
     fitted |>