Minor issue, fix warning, use tibble::as_tibble

To avoid warning as `tbl_df` is depricated, use `as_tibble`:

```R
# R version 4.0.2 (2020-06-22), wakefield_0.3.6

r_data_frame(n = 5, dob)
# # A tibble: 5 x 1
#   DOB       
#   <date>    
# 1 2008-05-15
# 2 2007-03-23
# 3 2008-04-27
# 4 2008-01-21
# 5 2007-09-19
# Warning message:
# `tbl_df()` was deprecated in dplyr 1.0.0.
# Please use `tibble::as_tibble()` instead.
```




