Sort before uniq to return unique ISSNs

[mawds]

In the exercise to count the number of unique ISSN values, uniq only matches adjacent matching lines, so the list of ISSNs should be sorted before piping to uniq.

This PR notes that uniq only works on adjacent lines, and modifies the exercise to sort the list of ISSNs before piping through uniq

[danmichaelo]

Good catch! But note that -P is not available in BSD grep (as on Macs) :( So perhaps using number ranges [0-9] is the best cross-platform solution.

[weaverbel]

Are you able to amend the code to reflect that @danmichaelo ?