Fix @concept tags in documentation

[Robinlovelace]

As pointed out by CRAN maintainers:

Dear maintainer,

You have several multi-line (and multi-paragraph) \concept entries in
your Rd files, e.g.

---

\concept{example destinations

This dataset represents trip destinations on a different geographic
level than the origins stored in the cents.}

---

in man/destination_zones.Rd. This is not as intended: each \concept
entry should really only give a single index term (word or phrase) in a
single line of plain text, as in the examples given in R-exts.

Please change your \concept entries accordingly (R-devel may soon have
problems building the reference manual for such entries, as they will be
included in the package manual index).

Please fix before 2019-05-31 to safely retain your package on CRAN.