Clarify 'How do you find the length of a string?' exercise

[anyweez]

https://github.com/tiy-tpa-fee/koans/blob/master/koans.original.js#L192

Just had a student (@encryptid) point out that this would be clearer if the name (fruit) and value ('apple') of the variable didn't have the same number of letters.

@ambethia you cool if we submit a PR to change that?

[ambethia]

@anyweez @encryptid I'd love it! Please do!