remove return value of GetApproximateSizes in index.md

The documentation (README.md and index.md) referred to the
public headers using an incorrect path - fixing.

PiperOrigin-RevId: 252922925

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=197452015

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