Don't call non-vectors "vectors"

[rossellhayes]

Remove messages with "vector" from the fallback method in friendly_class().
Now if the method falls back, it will always describe the object as an "object".
All atomic vectors types already have an associated friendly class message, so they will still be correctly described as vectors.

.result <- 1:2
.solution <- lm(mpg ~ wt, data = mtcars)
tbl_grade_class()
#> <gradethis_graded: [Incorrect]
#>   Your result should be an object with class `lm`, but it is a vector
#>   of integers (class `integer`).
#> >

.result <- 1:2
.solution <- c("1", "2")
tbl_grade_class()
#> <gradethis_graded: [Incorrect]
#>   Your result should be a vector of text (class `character`), but it is
#>   a vector of integers (class `integer`).
#> >

^{Created on 2022-08-09 by the reprex package (v2.0.1)}

Closes #120.