Fixing v2 api example

[OdairTrujillo]

I am working with the Uniswap v2 documentation because is the doc recommended by PancakeSwap v2 documentation that lacks of formal and complete docs like Uniswap have.

I think that this effort wort because the devs that are starting with PancakeSwap v2 need that the most part of the Uniswap v2 docs works.