Fix tibble-like printing in vignette

[urswilke]

in https://github.com/r-lib/pillar/blob/main/vignettes/extending.Rmd, the statement in

pillar/vignettes/extending.Rmd

Line 44 in 4d8d4d0

The `"default"` class doesn't have any customizations yet, and prints like a regular tibble.

is not correct (it doesn't print like a regular tibble).

If I pass NULL instead of "default" it works:

library(pillar)

example_tbl <- function(class) {
    vctrs::new_data_frame(
        list(
            a = letters[1:3],
            b = data.frame(c = 1:3, d = 4:6 + 0.5)
        ),
        class = c(class, "tbl")
    )
}
example_tbl(NULL)
#> # A data frame: 3 × 2
#>   a       b$c    $d
#>   <chr> <int> <dbl>
#> 1 a         1   4.5
#> 2 b         2   5.5
#> 3 c         3   6.5