Correct stride pseudocode in p27

[dunnoconnor]

This pull request updates the reduction loop in the block-wide reduction examples for both the English and Korean versions of Puzzle 27. The main change is replacing the Python for loop with a while loop that halves the stride at each iteration, making the reduction logic clearer and more idiomatic for parallel programming.

Reduction loop logic update:

• Replaced the for stride in range(64, 0, -1): loop with a while stride > 0: loop, initializing stride = 64 and halving it each iteration using stride //= 2, in both the English (book/src/puzzle_27/puzzle_27.md) and Korean (book/i18n/ko/src/puzzle_27/puzzle_27.md) documentation. [1] [2] [3] [4]