Update 06-auto-router.md

[webcaetano]

This fix a code bug, because ChainId were never called

Also the code just above

const WETH = new Token(
  1,
  '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2',
  18,
  'WETH',
  'Wrapped Ether'
);

Is also like this

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[pjona]

We could import ChainId by changing this line:

import { AlphaRouter } from '@uniswap/smart-order-router'

into:

import { AlphaRouter, ChainId } from '@uniswap/smart-order-router'

And then probably it's better to use ChainId.MAINNET for WETH token.

[adjkant]

Closing in favor of #420