Fix numbering of challenge exercises

[maneesha]

How could the content be improved?

This page contains several challenges. Four are numbered but there is also an un-numbered challenge. Recommend re-numbering so all challenges are numbered (making it go from 1 to 5).

Which part of the content does your suggestion apply to?

https://datacarpentry.org/r-intro-geospatial/03-data-structures-part1.html