Update to fix crypto text normalization.

This would update the readme for this exercise to be consistent with the source problem and to keep the idea of "square" more exact.  If this is changed, then the following change should be rejected:
exercism/xjavascript#61

If this is rejected, that change should be accepted.
This doesn't look like the correct chunk size to me. The plaintext version is in chunks of 8, this is chunks of 7 except for the last two chunks which are 6. That's inconsistent with other behavior that I've seen.