README.md: Fixed typos.
Week selector rework. Using ISO week dates. Closes #46.

Not sure what the week selector was doing before this rework. I asked
the author but it looked that he does not know either (at least when I
asked, a couple of years after he wrote the code) …
https://github.com/AMDmi3/opening_hours.js/issues/18

See https://en.wikipedia.org/wiki/ISO_week_date