fix: address base R example in readme does not work

In README, we have the following example, which isn't working:

```r
library(maidr)

# Create plot first
barplot(
  table(mtcars$cyl),
  main = "Cars by Cylinder Count",
  xlab = "Cylinders",
  ylab = "Count"
)

# Then call show() without arguments
show()
```