crypto-square: Clarify rectangular output requirement
crypto-square: Clarify rectangular output requirement

The proposed text is clearer than the original for two reasons:

* It makes clear that the chunks are unconditionally of `r` length. The
  previous text didn't make clear whether "of `r` length" only applied
  to "Phrases that fill perfect rectangles" or all phrases.
* "empty spaces" had an unclear correspondence to the space character.

Ambiguity in the original wording, especially "empty spaces", has led to
much confusion.

https://github.com/exercism/problem-specifications/pull/1210

exercism/xhaskell