fix: add a </pre> to error-handling.md

[andrwsalcdo]

#3446

Checklist:

• If this PR is a new feature, please 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
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[apollo-cla]

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[hwillson]

Thanks very much @andrwsalcdo!

[andrwsalcdo]

@hwillson No Problem