Fixing bug with `slice_tail(n())`

[JosephCrispell]

The following changes seem to fixed the errors and drake::r_make() is now running to completion:

• Changed slice_tail(n()) at line 16 in filterA2F.R to slice_tail() - by default slice_tail() assumes n=1
• Changed slice_tail(n()) at line 17 in filterJobs.R to slice_tail()

drake_make() runs to completion (following installation of a couple fo LaTeX libraries, that R prompted with popup for me to install) with two warning messages:

Warning messages:
1: Problem with `mutate()` input `centroid`.
i st_centroid does not give correct centroids for longitude/latitude data
i Input `centroid` is `st_centroid(geometry)`. 
2: In st_centroid.sfc(geometry) :
  st_centroid does not give correct centroids for longitude/latitude data

[charlesbaillie]

updated with suggested edit