Fix a small typo in comment.

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Merge pull request #14860 from twbs/comment-typo

Fix a small typo in comment.
Docs: Use a 180x180 apple-touch-icon.png.

Refs:
https://github.com/h5bp/html5-boilerplate/commit/1c4fcf32142e888946db579df0efbb52542e0ed1
https://github.com/h5bp/html5-boilerplate/commit/345925f04384b59929d8d39ba428cef548921408

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