Adjust Fetcher interface in docs

[zcei]

As mentioned in #891, the docs for a custom fetcher are off since a couple of months.

After c91e577 landed, the types were changed to accept a DocumentNode instead of a string.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[apollo-cla]

@zcei: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[JosephCarrington]

Thanks so much for this!

[zcei]

@freiksenet maybe? You merged the PR that introduced this behavior as well 😛

[hwillson]

Looks great @zcei - thanks!