Update migrating-from-v1-to-v2.md

[pavanpodila]

Added a note around possible wrong import of graphql from the graphql package instead of gatsby. I was bitten by this badly and took more than an hour to figure out!

[m-allanson]

Eeee that's no fun, thanks for the update @pavanpodila 👍 This is the sort of thing we could catch with #7687

[gatsbot]

Holy buckets, @pavanpodila — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

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If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!

[pavanpodila]

Eeee that's no fun, thanks for the update @pavanpodila 👍 This is the sort of thing we could catch with #7687

The eslint-plugin would be a perfect fit!!