Update go docs (#56)

I'm guessing this version (from the CGO example) is more widely applicable (I ship a `database/sql` driver as well).

PS: if you find that all the copies and allocs are a bottleneck, I'm guessing an `buf, err := sqlite_vec .AppendFloat32(buf, values)`, or even an API change (on my side) that allows serializing directly into "native" memory, would help.