Fix index.md

[jl0x61]

Remove return value of GetApproximateSizes in index.md.
The return type of DB::GetApproximateSizes is void:

virtual void GetApproximateSizes(const Range* range, int n,
                                 uint64_t* sizes) = 0;

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[jl0x61]

@googlebot I signed it!

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CLAs look good, thanks!

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[cmumford]

Nice catch @jl0x61. I'll get this merged.

[cmumford]

Merged. thx.