Polish READMEs for datasets

[zelima]

Some of the READMEs are outdated or not quite exact in their descriptions Eg: https://github.com/datasets/country-list says it's update while actually, it is not.

This list is updated whenever a change to the official code list in ISO 3166-1 is effected by the ISO 3166/MA.

Tasks

• [x] List core datasets
• [x] Review all core datasets and their READMEs and identify if any of them needs to be updated
• [x] Cross check with https://github.com/datasets/registry/blob/master/core-list.csv
• [x] List the ones that need
• [x] Update which necessary

[Branko-Dj]

FIXED!