chore(gatsby-source-graphql): Update README url => uri (#30872)
chore(gatsby-source-graphql): Update README url => uri (#30872)

* Update README.MD

After spending too much time figuring out why i was getting a `only absolute urls are supported` from `node-fetch` it was because of a typo in the docs. The `uri`'s fallback is set to `/graphql`.

* Update README.md

Co-authored-by: Lennart <074ba1c0efc36927866875b17be4fa06eb80e472@gmail.com>
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