The default size was changed in #f779e7a5 but the documentation was
never updated.

This fixes #566 reported on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194547959

Markdown is more readable in a text editor and when hosted
on GitHub is more readable than HTML.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148830423