Replace leafs with leaves to appease Spell Check with Typos workflow

[westonruter]

Summary

Fixes issue uncovered by @aaronjorbin in #964

Relevant technical choices

English: s/leafs/leaves/g

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Props: westonruter, jorbin, mukesh27.

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: aaronjorbin <jorbin@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[aaronjorbin]

I've been waiting my entire WordPress Contribution career to be asked to review someone else's spelling! Thanks @westonruter!